MAIN  LIBRARY-AGRICULTURE 


AGRICULTURAL  AND  BIOLOGICAL  PUBLICATIONS 
CHARLES  V.  PIPER,  CONSULTING  EDITOR 


BREEDING  CROP  PLANTS 


cMz  Qraw-MlBock  &  1m 


PUBLISHERS     OF     BOOKS      FOP^ 

Coal  Age     v     Electric  Railway  Journal 

Electrical  World  v  Engineering  News -Record 

American  Machinist  ^  Ingenieria  Internacional 

Engineering  8  Mining  Journal      ^     Po we  r 

Chemical  6   Metallurgical  Engineering 

Electrical  Merchandising 


oiiliiiiiiiiiiiiiiiiiiiHimillllllllniiiiiiiiiiiiiiiiniiiiiiiiiiiiMiiiiHiiniiiiii 


«  0 

11 

ll 


a  ® 

i3    O 
03     2 

II 

B 

s 


BKEEDING 

CROP  PLANTS 


BY 
HERBERT  KENDALL  HAYES 

PROFESSOR   OF   PLANT  BREEDING,   COLLEGE   OF   AGRICULTURE, 
UNIVERSITY   OF   MINNESOTA 

AND 

RALPH  JOHN  GARBER 

FORMERLY   ASSISTANT   PROFESSOR   OF   PLANT  BREEDING,   COLLEGE   OF   AGRICULTURE; 
UNIVERSITY    OF    MINNESOTA",    NOW     ASSOCIATE    PROFESSOR    AND    HEAD    OF,  »' 
THE   DEPARTMENT   OF   AGRONOMY,   UNIVERSITY   OF   WEST  VIRGINIA^  '  ' 


FIRST  EDITION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW   YORK:    370  SEVENTH  AVENUE 

LONDON:    6  &  8  BOUVERIE  ST.,  E.  C.  4 

1921 


S3 
H3 


COPYRIGHT,  1921,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


-Y-ASRIGOL.TXI 


THE  MAPI.E  PRESS  YORK: 


Z3o 
EDWARD  MURRAY  EAST 


465031 


PREFACE 

Since  the  early  development  of  agriculture  by  primitive 
peoples,  selection  of  seed  for  planting  has  been  an  important 
feature  of  agricultural  practice.  While  many  of  our  better 
varieties  or  strains  of  crop  plants  have  originated  as  chance 
seedlings  or  from  selections  made  by  men  who  lacked  a  knowledge 
of  the  laws  of  heredity,  there  has  been  a  growing  appreciation 
in  recent  years  of  the  value  of  training  students  for  the  occupa- 
tion of  plant  breeding. 

Studies  in  crop  genetics  carried  on  since  1900,  as  well  as 
studies  in  field  plot  technic,  have  helped  in  a  large  measure 
to  standardize  methods  of  breeding.  Information  regarding 
the  mode  of  inheritance  of  particular  characters  as  well  as  a 
better  knowledge  of  the  wild  relatives  of  our  crop  plants  is  con- 
stantly being  obtained.  The  purpose  of  this  book  is  to  present 
fundamental  principles  of  crop  breeding  and  to  summarize 
known  facts  regarding  the  mode  of  inheritance  of  many  of  the 
important  characters  of  crop  plants.  Much  of  the  material 
here  presented  has  been  used  in  courses  in  crop  breeding  which 
have  been  given  in  recent  years  at  the  College  of  Agriculture, 
University  of  Minnesota. 

Suggestions  from  others  in  relation  to  methods  of  treatment 
of  various  subjects  have  been  of  material  value.  Particular 
mention  should  be  made  of  the  helpful  advice  of  Dr.  M.  J. 
Dorsey  regarding  the  chapters  on  " Plant  Genetics"  and  " Fruit 
Breeding;"  of  F.  A.  Krantz  regarding  the  chapter  on  "Potato 
Breeding,"  and  of  John  Bushnell  and  W.  T.  Tapley  regarding 
the  chapter  on  "Vegetable  Breeding." 

We  are  also  indebted  to  Miss  Alice  McFeely,  Bulletin  Editor, 
for  many  suggestions  regarding  presentation  and  for  assistance 
in  proofreading;  to  Mr.  A.  N.  Wilcox  for  assistance  in  proof- 
reading; to  Miss  L.  Mae  Centerwall  for  help  in  obtaining  a  con- 
siderable number  of  publications  from  other  libraries;  and  to 
Miss  Alma  Schweppe  for  checking  the  literature  citations.  Pre- 
vious summaries  of  certain  phases  of  plant  breeding  methods  were 

IX 


x  PREFACE 

made  available  through  the  kindness  of  Professor  Andrew  Boss. 
The  many  helpful  suggestions  made  by  Dr.  C.  V.  Piper,  Consult- 
ing Editor  of  these  publications  have  been  of  great  value. 

Several  illustrations  have  been  supplied  by  investigators  who 
have  made  intensive  studies  of  particular  crops;  credit  for  these 
has  been  given  in  connection  with  the  illustrations.  Most 
of  the  other  figures  are  from  photographs  by  T.  J.  Horton, 
official  photographer  at  University  Farm,  St.  Paul.  Figures 
on  flower  structure  are  from  drawings  made  by  G.  D.  George, 
illustrator. 

The  papers  of  many  investigators  have  been  referred  to  in 
the  text,  as  the  advanced  student  will  frequently  desire  to 
study  the  original  publication.  The  possibilities  of  errors  are 
very  great  in  a  text  which  reviews  the  studies  of  numerous 
investigators.  The  writers,  therefore,  earnestly  invite  the 
criticism  of  the  readers. 

THE  AUTHORS. 

UNIVERSITY  OF  MINNESOTA, 
June,  1921. 


CONTENTS 


CHAPTER  I 
INTRODUCTION 

PAGE 

The  founders  of  the  art  of  plant  breeding 2 

The  first  demonstration  of  sex  in  plants 3 

Further  proof  of  plant  sexuality 4 

The  studies  of  Koelreuter 5 

Early  studies  of  the  cytology  of  fertilization 5 

An  answer  to  the  question  of  hybrid  fertilization 5 

The  great  hybridist  Gartner 6 

Early  English  plant  breeders 6 

Other  workers  of  this  period 7 

The  relation  of  certain  biologic  principles  to  plant  breeding 8 

The  doctrine  of  the  constancy  of  species 9 

Darwin's  theory  of  natural  selection 9 

The  stability  of  the  germ  plasm 10 

DeVries'  mutation  theory 11 

The  pure-line  theory 11 

Mendel's  law  of  heredity 12 

Hybridization  as  a  means  of  producing  variations 13 

The  value  of  crop  improvement  in  relation  to  a  more  efficient  agriculture  14 

CHAPTER  II 
PLANT  GENETICS 

Methods  of  studying  inheritance  of  characters 16 

The  mode  of  sexual  reproduction  in  flowering  plants 17 

The  inheritance  factors 20 

Variability  of  characters 20 

A  cross  in  which  the  parents  differ  by  a  single  factor 22 

Inheritance  of  two  independently  inherited  characters 23 

Several  factors  necessary  for  the  production  of  a  character 25 

Linkage  of  characters  in  inheritance 26 

Inheritance  of  quantitative  characters 27 

Stability  of  inherited  factors 32 

CHAPTER  III 
THE  MODE  OF  REPRODUCTION  IN  RELATION  TO  BREEDING 

Natural  crossing  with  self -fertilized  plants 34 

Wheat,  34;  Barley,  Oats,  Tobacco,  Flax,  36;  Rice,  37;  Cotton, 
Grain  Sorghums,  Peas  and  Beans,  38;  Tomatoes,  39. 

The  often  cross-pollinated  plants 39 

Maize,  Rye,  39;  Alfalfa,  40;  Grasses,  41. 

xi 


xii  CONTENTS 

PAGE 

Effects  of  a  cross  in  normally  self -fertilized  species 41 

Effects  of  self-fertilization  in  normally  cross-fertilized  plants    ....  45 

Explanation  of  hybrid  vigor 47 

CHAPTER  IV 

FIELD  PLOT  TECHNIC 

Soil  heterogeneity ,.    „    .    .-"..    .  51 

Harris'  method  of  estimating  soil  heterogeneity .   ........  51 

Estimating  soil  heterogeneity  by  means  of  checks .....',..  53 

Use  of  checks  in  correcting  yields.    .    .    .    .    .    .   -    .    -.-  ~.  V  .    .    •  53 

Use  of  probable  error  in  eliminating  strains .-   .  r -.  \.  .    .  57 

The  pairing  method  of  securing  a  probable  error  .    .    .  \    „    /  .-   .  57 

Replication  and  its  value •  ."  .  58 

Size  of  plot '•.,'.    .  ".    .....    .    .\t   ,    .  60 

Shape  of  plot  and  border  effect .....!*.  61 

Competition  as  a  factor  in  plot  variability 62 

Climatic  variations 64 

Summary  of  field  plot  technic 65 

CHAPTER  V 

CONTROLLING  POLLINATION    * 

Selfing  plants  artificially 67 

Technic  of  crossing .......  68 

Crossing  of  smal'  grains '. , 69 

Crossing  large-flowered  legumes ..../.  *   .    .  71 

Depollination  with  water 73 

Summary  of  technic  of  crossing 74 

CHAPTER  VI 
CLASSIFICATION  AND  INHERITANCE  IN  WHEAT 

Genetic  classification .-  .    .    .  75 

Wheat  species  groups TV...  77 

Poloni  um  crossed  with  other  species 79 

Some  linkage  results  in  wheat  crosses '....?-...  79 

Spike  density ..;....;....    .    .  81 

Seed  characters    .    .    ...    .    .    .'  v  .    ..  .*.".'  ..........  81 

Chaff  characters . 84 

Presence  or  absence  of  beards 85 

Inheritance  of  disease  resistance  .    ..•..-. 85 

Inheritance  of  other  characters 87 

CHAPTER  VII 
CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS  OTHER  THAN  WHEAT 

Classification  and  inheritance  in  oats 89 

Crosses  betwen  Avena  fatua  and  A.  sativa 90 

Origin  of  cultivated  varieties  of  A.  sterilis 90 

Differences  in  awn  development 90 


CONTENTS  xiii 

PAGE 

Color  of  grain  and  straw I  .  ; 92 

Hulled  versus  hull-less 93 

Pubescence 94 

Characters  of  base  of  lower  grain 94 

Open  versus  side  panicle 95 

Resistance  to  rust 95 

Size  characters 96 

Linkage  of  characters 97 

False  wild  oats 97 

Classification  and  inheritance  in  barley 98 

Species  crosses 99 

Simple  Mendelian  characters 102 

Winter  versus  spring  habit 103 

Density  of  the  spike 103 

The  barley  awn  in  relation  to  yield  104 

Some  rye  studies 106 

Wheat-rye  hybrids 106 

Buckwheat 107 

Breeding  buckwheat 108 

Rice 108 

Inheritance  of  characters 108 

CHAPTER  VIII 

METHODS  OF  BREEDING  SMALL  GRAINS 

Method  of  keeping  continuous  records Ill 

New  introductions 113 

Select'on 114 

Summary  of  methods  of  selection 115 

Crossing 116 

Technic  of  harvesting,  thrashing,  etc 117 

CHAPTER  IX 

SOME  RESULTS  OF  SELECTION  WITH  SELF-FERTILIZED  CROPS 

Early  investigators  in  selection  of  self -fertilized  cereals 118 

Selection  within  a  pure  line 120 

Selection  for  the  purpose  of  isolating  a  pure  line 125 

Wheat  selections 128 

Oat  selections .    .  129 

Selections  in  other  self -fertilized  crops 131 

CHAPTER  X 

SOME  RESULTS  OF  CROSSING  AS  A  MEANS  OF  IMPROVING  SELF-FERTILIZED 

CROPS 

The  improvement  of  black  oats  at  Svalof  .    .    .    .  ...    . -.    „    .    .    .    .    .  132 

A  wheat  cross  made  at  Svalof ^  ......  134 

Wheat  breeding  at  University  Farm,  Cambridge,  England ......  134 

Farrer's  wheat  breeding  in  Australia .135 


xiv  CONTENTS 

PAGE 

Marquis  wheat .- .    .    .s.   . 136 

Winter  wheat  breeding  at  the  Minnesota  Experiment  Station  .    .    .    .137 

Breeding  beans  resistant  to  Colletotrichum  lindemuthianum 139 

An  improved  strain  of  tobacco .....'....  139 

Summary 142 

CHAPTER  XI 
COWPEAS,  SOYBEANS  AND  VELVET  BEANS 

Cowpeas  (Vigna  sinensis) .    .......  143 

Origin .    . 143 

Description  and  inheritance 143 

Some  results  of  selection  and  crossing 145 

Soybeans  (Soja  max)  ...'.- 146 

Origin 146 

Classification  and  inheritance 146 

Breeding 148 

Velvet  bean  (Stizolobium) 149 

Origin 149 

Important  characters  and  inheritance 149 

Mutations 150 

Breeding 151 

CHAPTER  XII 
FLAX  AND  TOBACCO 

Flax 153 

Species  crosses 153 

Inter-relation  of  factors  for  flower  and  seed  colors 153 

Inheritance  of  size  characters 156 

Wilt  resistance  in  flax 157 

Methods  of  breeding 158 

Tobacco 159 

The  genus  Nicotiana 159 

Parthenogenesis . 160 

Sterility • 160 

Color  characters «-.,......%...  v   .  161 

Quantitative  characters .    .    .    .  c.    .    .'..    .-  ;    .    .    .  162 

Environment  as  a  factor  in  tobacco  breeding 165 

Mutations  in  tobacco ."-.... 167 

CHAPTER  XIII 
COTTON  AND  SORGHUM 

Cotton .   ;  -i    .    .  173 

Classification  and  inheritance    .    .    .  " 173 

Mutations  in  cotton    .    .-..-.    .    .    ,    .    .    .    .    .    .«'..„•    .    .  177 

Cotton  breeding .;,... 177 


CONTENTS  xv 

PAGE 

Sorghum    ......./..; » 178 

Origin .     *  .    .    .-'..*•• 178 

Classification  and  inheritance 178 

Some  results  of  selection 179 

Methods  of  breeding  sorghum   . 179 

CHAPTER  XIV 
MAIZE  BREEDING 

Origin  and  species 181 

Inheritance  of  characters 183 

Endosperm  characters 183 

Plant  characters 187 

Colors  in  plant  organs 187 

Podded  condition 189 

Auricle  and  ligule 189 

Chlorophyll  inheritance 190 

Some  seed  and  ear  characters 192 

Size  characters 192 

Chemica  composition 193 

Corn  improvement  by  the  trained  plant  breeder 196 

Relation  of  ear  characters  to  yield 197 

Ear-to-row  breeding 198 

Home-grown  seed 199 

Relation  between  heterozygosis  and  vigor 200 

Immediate  effect  of  crdssing  on  size  of  seed 201 

FI  varietal  crosses 202 

Isolation  of  homozygous  strains .  205 

CHAPTER  XV 
GRASSES,  CLOVER  AND  ALFALFA 

Grasses 207 

Breeding  timothy . 210 

Clovers 214 

Red  clover 214 

Selection  for  disease  resistant  clover 215 

Alfalfa 215 

Grimm  alfalfa  and  winter  hardiness .    ..'..' .    .    .216 

CHAPTER  XVI 
POTATO  IMPROVEMENT 

Origin  and  species .    . .    ,   ... v  .    .  219 

Inheritance     ...".... 221 

Production  of  new  forms      . .    .   i  .  223 

The  difficulties  of  obtaining  crossed  seed 224 

Improvement  through  seedling  production 226 

Clonal  selection  .  .  228 


xvi  CONTENTS 

CHAPTER  XVII 

BREEDING  OF  VEGETABLES  PAGE 

Self -fertilized  vegetables 234 

Origin  of  vegetables 234 

Peas 236 

Some  classification  characters 236 

Inheritance 236 

Beans 241 

Some  classification  characters 241 

Inheritance 242 

Tomato 245 

Classification  characters  and  inheritance 245 

Peppers .  246 

Classification  characters  and  inheritance 246 

Methods  of  breeding  self-fertilized  vegetables 247 

Cross-fertilized  vegetables 248 

Radish;  origin,  inheritance  and  breeding 249 

Beets;  inheritance  and  breeding 250 

Cultivated  vegetables  of  the  genus  Brassica 251 

Inheritance 251 

Breeding .  ., 252 

Asparagus;  rust  resistance 253 

Economic  Cucurbitaceae 254 

Introduction  and  classification 254 

Immediate  effect  of  pollination . 256 

Cucumber 257 

Muskmelon.  . 257 

Squashes  and  gourds 258 

Watermelon 258 

Breeding  Cucurbitaceae 259 

CHAPTER  XVIII 
FRUIT  BREEDING 

Origin  and  antiquity  of  some  fruits .261 

Some  early  studies  of  fruit  improvement 264 

Von  Mons 264 

Knight -...../.  ; 264 

American  pomology 265 

Some  considerations  of  fruit  breeding .    .    .    .    .  265 

Overcoming  soil  heterogeneity 266 

Self-sterility  and  heterozygosity 267 

Inheritance  of  some  characters 271 

Apple..    ....,•'....'.- .">•  *    ;."-.  271 

Raspberry ....'.., 272 

Grape 272 

Illustration  of  methods  of  breeding 273 

Selection  of  bud  sports 273 

Controlled  crosses ' 277 


CONTENTS  xvii 

CHAPTER  XIX 

FARMERS'  METHODS  OF  PRODUCING  PURE  SEEDS 

PAGE 

Determination  of  better  varieties .  ' 281 

What  is  good  seed?.    ........  \    ..-..; 282 

Adaptability.    .    .    .  v. 282 

Yielding  ability  and  quality 282 

Purity 283 

Hardiness 283 

Strength  of  stalk 283 

Disease  escaping  or  resistance 283 

Methods  of  seed  production 284 

Seed  growers'  methods  for  self-fertilized  crops 284 

Improved  corn  seed.  ^ 287 

Method  of  breeding  corn  for  special  breeders .  ^. 288 

Method  of  corn  bleeding  for  average  farmer  ^ .    .    .  290 

Potato  seed  (tubers)  selection 290 

Improvement  by  selection  of  such  crops  as  alfalfa,  clover  and  grasses  292 

Seed  registry  or  certification 293 

DEFINITIONS 294 

LITERATURE  CITATIONS 299 

INDEX.                                                                                                            .  319 


BREEDING  CROP  PLANTS 

CHAPTER  I 
INTRODUCTION 

The  origin  and  mode  of  development  of  nearly  all  of  our 
principal  cultivated  crops  is  an  obscure  and  much  debated  sub- 
ject. This  is  partly  due  to  the  fact  that  many  crops  have  been 
grown  for  hundreds  of  years  and  often  the  same  forms  are  culti- 
vated as  were  grown  in  early  periods.  It  is  very  probable,  for 
example,  that  the  men  of  the  old  stone  age,  50,000  years  ago,  had 
some  sort  of  art  of  agriculture  (Dettweiler,  1914).  These  con- 
clusions have  been  drawn  from  old  engravings  of  this  period 
which  were  made  on  cavern  walls.  Wheat  and  barley  were  cer- 
tainly grown  in  early  times.  A  carving  of  the  upper  Paleolithic 
age  in  the  Pyrenees  mountains  shows  winter  barley  such  as  is  now 
cultivated  in  that  locality. 

Dettweiler  writes  very  interestingly  of  the  agriculture  of  the 
Lake  Dwellers  who  lived  during  the  period  from  4,000  to  2,000 
years  B.C.  He  states  that  the  Lake  Dwellers  of  Switzerland 
cultivated  the  short-eared,  six-rowed  barley,  Hordeum  sanctum  of 
the  ancients;  the  dense-eared,  six-rowed  variety,  H.  hexastichon 
L.,  variety  densum;  two-rowed  barley,  H.  distichon;  small  lake- 
dwelling  wheat,  Triticum  vulgare  antiquorum;  true  Binkel  wheat, 
T.  vulgare  compactum;  Egyptian  or  English  wheat,  T.  turgidum, 
L.;  an  awnless  thick-eared  emmer,  T.  dicoccum,  Schrank;  one- 
grained  wheat,  T.  monococcum,  L.;  meadow  (common)  millet, 
Panicum  miliaceum,  L.;  club  millet,  P.  italicum,  L.;  and  a  type  of 
flax,  Linum  angustifolium,  which  still  grows  wild  in  Greece.  An 
excavation  was  made  in  the  village  of  Gleichberg,  near  Romhild 
in  1906.  On  an  old  fireplace,  with  remains  of  the  oldest  Bronze 
age,  were  found  the  following  seeds:  einkorn,  spelt,  binkel,  and 
small  lake-dwelling  wheat,  small  lake-dwelling  barley,  vetch, 
peas,  poppy,  and  possibly  apple  seeds. 

It  is  not  the  purpose  to  give  the  historical  development  of 
crops  except  to  show  that  many  were  cultivated  in  very  ancient 

1 


2  BREEDING   CROP  PLANTS 

times  by  primitive  peoples  who  developed  many  varieties.  As 
some  of  the  varieties  which  were  then  grown  are  in  existence 
today  and  are  cultivated  in  some  regions,  a  little  idea  of  earlier 
work  is  obtained. 

Coming  now  more  nearly  to  present  times  we  may  briefly 
consider  the  work  of  the  Indians  with  maize.  Piper  speaks 
of  the  plan  by  which  seeds  of  different  colors  were  planted 
together  in  one  hill  with  the  thought  that  this  method  gave 
increased  yields.  It  tended  to  keep  the  varieties  in  a  heterozy- 
gous condition.  During  the  last  three  or  four  years  Squaw  Flint 
from  the  Indian  reservations  in  Minnesota  has  averaged  as  large 
a  yield  per  acre  at  University  Farm,  St.  Paul,  as  the  more  care- 
fully selected  varieties. 

These  facts  should  help  to  give  the  student  of  plant-breeding 
some  idea  of  the  great  accomplishments  in  plant  production  in 
earlier  times  and  to  correct  possible  exaggeration  of  relative  values 
of  the  results  of  recent  work.  Present-day  breeding  has  achieved 
great  results  and  will  accomplish  much  more;  the  foundation, 
however,  was  laid  many  years  ago. 

THE  FOUNDERS  OF  THE  ART  OF  PLANT  BREEDING 

The  relation  between  the  science  and  the  art  of  plant  breeding 
is  a  very  interesting  subject.  Through  many  years  of  trials, 
methods  are  improved;  and  a  correct  knowledge  of  the  funda- 
mentals of  the  science  often  does  not  widely  modify  the  actual 
practice  involved.  As  a  rule,  scientific  principles  allow  some 
short  cuts  in  breeding  methods  and  help  to  eliminate  erroneous 
and  useless  practices. 

As  will  be  constantly  emphasized  in  this  work,  there  is  a  close 
relation  between  the  mode  of  reproduction  and  the  methods  of 
breeding  a  plant.  A  knowledge  of  sexuality  was,  therefore, 
almost  a  necessity  before  it  was  possible  to  develop  the  art  of 
breeding.  Sexual  processes,  while  not  thoroughly  understood, 
were  observed  in  animals  three  or  four  centuries  B.C.  by  the 
Egyptians  and  Assyrians.  Existence  of  fruit-bearing  and  sterile 
trees  of  the  date  palm  was  known  to  the  people  of  Egypt  and 
Mesopotamia  in  early  times  and  records  of  artificial  pollination 
as  early  as  700  years  B.C.  have  been  found  (see  Fig.  1).  The 
Assyrians  commonly  referred  to  the  date  trees  as  male  and 
female.  The  Greeks,  however,  to  whom  we  look  for  early 


INTRODUCTION  3 

scientific  thought,  failed  to  interpret  this  phenomenon.  Theo- 
phrastus,  for  example,  concludes  that  as  other  plants  do  not  as 
a  rule  exhibit  the  same  phenomenon,  the  date  tree  is  not  an 
example  of  real  sexuality  (Johnson,  1915). 

Little  was  actually  known  of  plant  sexual  processes  until 
comparatively  recent  times.  The  English  physician  Grew 
(1676)  further  developed  the  suggestion  of  Sir  Thomas  Millington 
that  the  stamens  served  as  the  male  organs,  by  a  hypothesis 
regarding  the  process  of  fertilization.  The  only  means  of 
demonstrating  this  phenomenon  was  by  the  experimental  method. 


FIG.   1. — The  date  palm  among  the  Assyrians. 

"Design  from  the  palace  of  Sargon  at  Khorsabad  (eighth  century  B.C.)  showing  that 
the  male  and  female  flowers  of  the  date  palm  were  clearly  distinguished  at  that  time.  The 
worshiper  in  the  middle  is  carrying  a  sprig  of  male  or  staminate  flowers  while  the  one  at 
the  right  bears  female  or  pistillate  blossoms.  The  drawings  should  be  compared  with  the 
photographs  of  actual  flowers.  The  winged  deity  at  the  left,  who  is  usually  identified  as 
the  Palm  God,  holds  in  his  hand  a  cone  which  is  thought  to  typify  the  spathe  of  the  male 
palm,  and  thus  the  principle  of  fertility  in  general."  (After  Johnson,  1915.) 

The  First  Demonstration  of  Sex  in  Plants. — Camerarius 
first  made  the  experimental  test  by  using  isolated  female 
plants  of  the  mulberry,  by  emasculating  the  castor  bean  and  by 
removing  the  stigmas  from  Indian  corn.  The  results  of  these 
experiments  were  reported  in  a  letter  to  Professor  Valentin,  of 
Giessen,  written  in  1694. 

The  following  statement,  made  by  Camerarius  and  found  in 
Ostwald's  Klassiker,  page  25,  has  been  frequently  quoted  (John- 
son 1915.) 


4  BREEDING  CROP  PLANTS 

"In  the  vegetable  kingdom  there  is  accomplished  no  reproduction 
by  seeds,  that  most  perfect  gift  of  nature,  and  the  usual  means  of 
perpetuating  the  species,  unless  the  previously  appearing  apices  of  the 
flower  have  already  prepared  the  plant  therefor.  It  appears  reasonable 
to  attribute  to  these  anthers  a  nobler  name  and  the  office  of  male  sexual 
organs." 

Further  Proof  of  Plant  Sexuality. — The  work  of  Camerarius 
was  confirmed  by  several  men.  Thomas  Fairchild,  in  1719, 
produced  a  new  variety  of  pinks  by  an  artificial  crossing;  of  two 


FIG.  2. — Male  and  female  flowers  of  date  palm  about  two  times  natural  size. 
(Photograph  taken  by  Swingle  in  Sahara  Desert,  1X99.) 

varieties;  and  Bradley,  two  years  earlier,  found  emasculated 
tulips  set  no  seed.  Miller,  1731,  noted  insects  pollinating  emas- 
culated tulips  after  first  visiting  untreated  tulip  flowers.  Gover- 
nor Logan  of  Pennsylvania,  in  1739,  experimented  with  maize 
and  observed  that  detasseled  plants  set  no  seed  when  isolated 
from  untreated  plants.  He  also  removed  the  silks  and  found 
such  treated  plants  were  incapable  of  setting  seed.  Gleditsch 


INTRODUCTION  5 

(1750)  had  a  pistillate  palm  in  Berlin  which  was  80  years  old 
and  had  set  no  seed.  He  obtained  a  quantity  of  pollen  from 
trees  in  Leipsic  (then  nine  days'  journey  from  Berlin)  and  after 
pollination  seed  was  produced  which  germinated. 

The  Studies  of  Koelreuter.1 — While  these  investigators  and 
others  confirmed  the  work  of  Camerarius,  little  advance  was  made 
in  the  art  of  breeding  until  Koelreuter  (1761)  made  a  careful 
study  of  artificial  crosses  and  gave  the  first  extended  account. 
In  tobacco  crosses,  for  example,  he  found  that  the  first  generation 
was  of  intermediate  habit  and  therefore  showed  the  effect  of  the 
male  parent.  His  work  on  the  vigor  of  first  generation  crosses 
is  of  much  interest.  He  believed  the  "oil"  of  the  pollen  grain 
after  mixing  with  the  stigmatic  fluid  penetrated  the  ovule.  The 
belief  of  a  union  of  male  and  female  substances  was  a  step  in 
the  right  direction.  The  value  of  insects  as  carriers  of  pollen 
was  also  demonstrated. 

Early  Studies  in  the  Cytology  of  Fertilization. — Pollen  tubes 
were  first  observed  in  1823  by  Amici  who  followed  them  to  the 
micropyle  of  the  ovule  in  1830.  Schleiden  shortly  afterward 
made  numerous  studies  of  the  pollen  tube  and  apparently  thought 
the  embryo  developed  in  the  embryo  sac  from  the  end  of  the 
pollen  tube.  This  matter  was  not  thoroughly  cleared  up  until 
Strasburger  (see  Johnson,  1915)  concluded,  in  1884,  that: 

"1.  The  fertilization  process  depends  upon  the  copulation  with  the 
egg  nucleus  of  the  male  nucleus  which  is  brought  into  the  egg.  2.  The 
cytoplasm  is  not  concerned  in  the  process.  3.  The  sperm  nucleus,  like 
the  egg  nucleus,  is  a  true  cell  nucleus. " 

An  Answer  to  the  Question  of  Hybrid  Fertilization. — Al- 
though Koelreuter  proved  the  fact  of  sexuality  in  plants  it 
was  not  generally  accepted,  and  early  in  the  nineteenth  century 
the  Physical  Section  of  the  Royal  Prussian  Academy  offered  a 
prize  for  an  answer  to  the  question,  "Does  hybrid  fertilization 
occur  in  the  plant  kingdom?"  Among  other  results  presented 
by  Weigmann  in  answer  to  this  question  occurs  the  statement  of 
the  immediate  effect  of  pollen  in  legumes.  Weigmann  made  a 
study  of  36  crosses  using  the  following  plants:  onion,  cabbage, 
pea,  bean,  lentil,  pink,  and  tobacco.  He  observed  the  fact  of 
variability  due  to  crossing  and  thought  gardeners  should  pay 

1  For  these  facts  the  papers  of  other  writers  have  been  freely  used.  Those 
by  ROBERTS  (1919)  have  been  especially  helpful. 


6  BREEDING  CROP  PLANTS 

more  attention  to  the  planting  of  their  crops  so  that  those  of  like 
kind  did  not  grow  so  near  each  other  that  crossing  through  the 
aid  of  insects  would  take  place.  Sprengel,  in  &  book  published  in 
1793,  showed  the  important  role  played  by  insects  in  pollination 
and  studied  the  adaptations  for  crossing  found  in  many  flowers. 
He  concluded  that  nature  intended  flowers  should  not  be  polli- 
nated by  their  own  pollen. 

The  Great  Hybridist  Gartner. — In  extent  and  number  of  his 
experiments  Gartner's  work  is  very  great.  In  1835  he  heard  of 
the  offer  of  a  prize  made  by  the  Dutch  Academy  of  Sciences  at 
Haarlem  regarding  the  place  of  hybridization  in  producing 
new  varieties  of  economic  and  ornamental  plants. 

Gartner's  paper  on  this  question,  which  received  the  prize,  was 
published  in  extended  form  in  1849.  He  made  thousands 
of  crosses,  involving  nearly  700  species,  and  obtained  about  250 
hybrids.  The  work  was  so  carefully  controlled  and  checked 
that  the  fact  of  sex  in  plants  was  thoroughly  proved.  He 
made  a  classification  of  hybrids  according  to  whether  they 
resembled  one  or  the  other  parent  in  all  respects,  whether  they 
resembled  one  parent  in  one  part  of  the  plant  and  the  other 
parent  in  some  other  characters,  or  whether  there  was  an  almost 
equal  balance.  In  the  last  case  in  later  generations,  the  inclina- 
tion toward  the  one  or  the  other  parent  was  supposed  to  be 
due  to  a  slight  overbalance  of  one  or  the  other  of  the  fertilizing 
materials.  Gartner  explains  the  appearance  of  the  first  hybrid 
generation  as  due  to  an  inner  force  operating  according  to  law. 
He,  like  Koelreuter  and  Weigmann,  observed  increased  vigor  in 
hybrids. 

He  made  experiments  to  determine  the  immediate  effect  of 
pollen  with  crosses  between  colorless  and  colored  pericarp 
varieties  of  maize  and  in  crosses  between  a  brown-seeded  Lychnis 
and  one  with  a  gray  seed.  As  no  change  occurred,  a  law  was 
developed  to  the  effect  that  pollen  does  not  immediately  affect 
forms  and  external  characters  of  seeds  but  influences  the  develop- 
ment of  the  resultant  plant.  He  observed  an  immediate  effect 
in  some  pea  crosses  and  learned  that  the  yellow  cotyledon  color 
dominated  the  green  in  the  hybrid  seeds. 

Early  English  Plant  Breeders. — Knight,  Goss,  and  Herbert, 
three  English  workers,  did  much  to  develop  the  art  of  breeding. 
Knight,  who  was  a  practical  horticulturist,  recognized  the  aid 
of  artificial  cross-pollination  in  producing  new  kinds.  He 


INTRODUCTION  7 

studied  the  question  of  the  immediate  effect  of  pollen.  A  variety 
of  pea  with  a  white  seed-coat  was  fertilized  with  pollen  of  a 
gray-seeded  variety.  No  immediate  influence  of  pollen  was 
obtained.  However,  when  the  resultant  plant  was  pollinated 
by  a  white  variety  both  gray-  and  white-seeded  sorts  were  ob- 
tained in  the  next  generation.  William  Herbert  was  a  contempo- 
rary of  Knight  who  learned  of  the  work  of  Koelreuter  some  time 
after  he  had  started  his  experiments.  He  opposed  the  idea  that 
species  crosses  were  necessarily  sterile. 

Studies  made  by  John  Goss  are  considered  of  much  interest  as 
they  showed  results  similar  to  those  obtained  later  by  Mendel. 
In  1820  flowers  of  Blue  Prussian  pea,  which  has  bluish  seeds, 
were  pollinated  with  pollen  of  Dwarf  Spanish.  Three  seeds  were 
obtained  which  were  yellowish- white  like  the  male  parent. 
Plants  grown  from  the  seeds  produced  some  pods  with  all  blue, 
some  with  all  white,  and  some  with  both  blue  and  white  seeds  in 
the  same  pods.  When  planted,  the  blue  seeds  bred  true  while 
the  white  seeds  gave  some  segregates.  No  law,  however,  was 
developed. 

Other  Workers  of  this  Period. — At  about  this  same  period 
Sargeret,  in  France,  was  making  studies  with  C ucurbitacece 
crosses.  He  observed  the  fact  of  dominance  as  the  following 
crosses  show. 

MUSKMELON  (FEMALE)         CANTALOUPE  (MALE)  FIRST  GENERATION 

1.  Flesh,  white  Flesh,  yellow  Flesh,  yellow 

2.  Seeds,  white  Seeds,  yellow  Seeds,  white 

3.  Skin,  smooth  Skin,  netted  Skin,  netted 

4.  Ribs,  slightly  evident    Ribs,  strongly  Ribs,  rather 

pronounced  pronounced 

5.  Flavor,     sugary     and  Flavor,  sweet  Flavor,  acid 

very    acid    at    same 
time 

He  notes  (Roberts,  1919)  that: 

"The  characters  were  not  blended  or  intermediate  at  all,  but  were 
clearly  and  distinctly  those  of  the  one  or  the  other  parent." 

Naudin  made  quite  careful  studies  and  attempted  to  summar- 
ize, his  results.  He  so  nearly  approached  the  law  later  laid 
down  by  Mendel  that  some-  workers  have  spoken  of  it  as  the 
Naudin-Mendel  law.  He  thought  that  if  hybrids  were  self- 
fertilized  they  would  return  more  or  less  rapidly  to  the  parental 
types.  Similar  results  were  obtained  if  the  hybrid  was  pollinated 


8  BREEDING  CROP  PLANTS 

by  one  of  its  parents.  He  noted  the  uniformity  of  the  first  gen- 
eration and  the  production  of  many  types  in  the  second  genera- 
tion some  of  which  could  not  be  told  from  the  original  parents. 
The  results  were  explained  by  the  segregation  of  specific  sub- 
stances in  the  pollen  and  ovaries  of  the  hybrid  (Naudin,  1865). 

Wichura  (1865)  found  reciprocal  crosses  gave  like  results  and 
therefore  concluded  that  the  pollen  and  the  egg  have  exactly 
the  same  share  in  the  organism  which  results  from  fertilization. 
He  studied  species  crosses  in  willows  but  did  not  deal  with  the 
individual  characters  of  the  species. 

Mendel's  work,  published  in  1866,  is  now  well  known  to 
all  students  of  genetics  and  plant  breeding.  This  early  paper 
remained  unnoticed  until  the  rediscovery  of  the  law  in  1900  by 
each  of  three  investigators,  DeVries,  Correns,  and  Tschermak. 
With  the  great  advances  made  since  that  time  rules  can  now  be 
given  which  furnish  a  reliable  guide  for  plant  breeding  operations. 
To  quote  from  Pearl: 

"In  the  creation  of  new  races  by  hybridization  the  plant  breeder  can 
and  does  take  Mendelian  principles  as  a  direct  and  immediate  guide. 
He  has  made  Mendelism  a  working  tool  of  his  craft." 

THE  RELATION  OF  CERTAIN  BIOLOGIC  PRINCIPLES  TO  PLANT 

BREEDING1 

The  art  of  plant  breeding  is  closely  related  to  those  biologic 
principles  which  furnish  the  foundation  for  the  science  of  breed- 
ing. For  this  reason  there  is  a  very  close  relation  between  the 
development  of  theories  of  evolution  and  scientific  methods  of 
breeding. 

The  conception  of  evolution  dates  from  the  time  of  the  Greek 
philosophers  in  the  eighth  century.  This  was  the  speculative 
period  and  evolutionary  beliefs  were  not  attained  as  a  result  of 
experimentation.  Until  the  sciences  of  botany  and  zoology  were 
built  up  it  was  impossible  to  do  more  than  outline  theories  which 
appealed  to  the  judgment  of  the  writer. 

The  modern  inductive  period  is  of  comparatively  recent 
times.  Erasmus  Darwin  developed  a  theory  of  evolution  which 
he  did  not  think  entirely  adequate.  Lamarck  gave  us  the  first 
well-rounded  theory  of  evolution.  It  was  based  on  the  inherit- 
ance of  acquired  characters.  By  continued  use  an  organ  was 

1  A  bulletin  by  EAST  (1907)  and  a  book  by  SCOTT  (1917)  have  helped 
materially  and  have  been  freely  used. 


INTRODUCTION  9 

strengthened  and  developed.  Likewise,  without  use  it  was  weak- 
ened. The  supposed  inheritance  of  these  acquired  characters 
was  the  basis  of  the  production  of  the  numerous  species. 

The  term  species  was  first  applied  to  animals  and  plants  by 
John  Ray  (1628-1705)  who  used  it  to  refer  to  a  group  of  organ- 
isms with  similar  characteristics  and  which  freely  intercrossed. 
Many  of  the  experiments  of  this  period  dealt  with  the  question 
of  species. 

The  Doctrine  of  the  Constancy  of  Species. — Linnaeus  (1707- 
1778)  adopted  a  more  strict  definition  although  he  was  not  always 
consistent  in  his  use  of  the  word.  The  doctrine  adopted  was  that 
of  the  separate  creation  of  fixed  entities  which  were  called  species. 
Lamarck  denied  this  theory  and  outlined  his  evolutionary 
hypothesis.  Most  naturalists  of  this  period  believed  in  the 
immutability  of  species. 

It  is  thought  that  the  work  of  Lyell  (1797-1875),  an  eminent 
geologist,  had  a  marked  effect  on  that  of  Charles  Darwin,  who  was 
his  intimate  friend.  Lyell  insisted  upon  the  continuity  of  the 
earth's  history  and  the  uniformity  of  agencies  which  wrought 
such  profound  changes  upon  the  earth.  This  theory  was  in 
opposition  to  that  of  Cuvier,  who  believed  that  the  earth's  history 
was  a  series  of  times  of  destruction  followed  by  periods  of  tran- 
quillity ("catastrophism").  After  each  such  destructive  period 
it  was  believed  that  new  .creation  took  place. 

Darwin's  Theory  of  Natural  Selection. — The  most  influential 
worker  in  the  history  of  development  of  the  evolutionary  con- 
ception was  Charles  Darwin.  He  and  Alfred  Russel  Wallace 
independently  developed  a  theory  for  the  origin  of  species  and 
united  in  presenting  a  preliminary  paper  in  1858. 

The  publication  of  Darwin's  " Origin  of  Species"  in  1859 
gradually  brought  about  a  belief  in  evolution.  The  work  of  Lyell 
had  helped  materially  to  develop  a  belief  in  the  orderly  progress 
of  the  world  and  assisted  in  preparing  the  way  for  the  masterly 
presentation  of  Darwin.  Darwin  presented  such  a  mass  of 
evidence  from  widely  different  fields  .that  the  entire  thinking 
world  was  compelled  to  accept  evolution  as  a  fact.  The  evidence 
was  grouped  under  such  headings  as  organic  relationship,  com- 
parative anatomy,  embryology,  paleontology,  and  domestication. 

The  fact  of  evolution  is  indisputable.  The  explanation 
is  even  yet  not  entirely  satisfactory.  Darwin's  theory  is  founded 
upon  a  series  of  facts  as  follows: 


10  BREEDING  CROP  PLANTS 

1.  Variability. — It  is  a  matter  of  common  observation  that 
no  two  individuals  are  exactly  alike.     If  sufficient  individuals 
are  examined  the  range  of  variation  is  found  to  be  quite  great. 
These  variations  are  universally  present. 

2.  A  Struggle  for  Existence. — If  all  the  progeny  of  some  of  the 
lower  forms  grew  to  maturity  and  each  in  turn  produced  as 
many  progeny,  the  world  would  soon  be  overrun  with  a  single 
form.     There  is  competition  also  between  different  species  and 
genera. 

3.  Natural  Selection. — The   conclusion  would  certainly  seem 
reasonable    that   those   forms   would   survive   which   possessed 
characters  better  adapted  to  a  given  environment  and  there- 
fore gave  those  particular  forms  advantage  in  the  struggle  for 
existence. 

4.  Heredity. — Variation    produces    the    material    for    natural 
selection  to  work  upon  and  heredity  tends  to  perpetuate  the 
variations. 

The  mechanism  of  transmission  of  characters,  the  physio- 
logical cause  of  variations,  and  the  inheritance  of  different- 
categories  of  variations  were  unanswered  problems.  Many 
criticisms  were  made  of  Darwin's  work  and  many  were  considered 
by  Darwin  himself.  Nearly  all  of  these  have  a  bearing  upon 
plant  breeding.  In  the  improvement  of  crops,  artificial  selection 
takes  the  place  of  natural  selection.  The  breeder  is  constantly 
faced  with  the  question  of  the  perpetuation  of  a  variation.  He 
also  faces  the  question  of  whether  the  useful  variation  will  per- 
petuate itself  in  crosses  or  will  be  lost. 

Darwin  recognized  two  sorts  of  variations,  the  " fortuitous" 
or  chance  variations,  i.e.,  those  which  are  everywhere  present  and 
which  cause  every  plant  to  be  slightly  different  from  other  plants 
of  the  same  species.  These  were  considered  to  be  of  primary 
importance  in  evolution.  While  he  recognized  "definite"  or 
discontinuous  variations,  the  so-called  mutations,  these  were  not 
considered  of  primary  importance. 

The  Stability  of  the  Germ  Plasm. — Weissmann's  theories  are 
of  much  interest.  He  developed  the  idea  of  the  continuity  of  the 
germ  plasm  and  that  external  agencies  could  not  modify 'inheri- 
tance without  first  affecting  the  germ  cells.  Plant  breeders  are 
not  particularly  interested  in  Weissmann's  ingenious  theories 
which  were  outlined  to  show  that  the  inheritance  of  acquired 
characters  was  an  impossibility.  Apparently,  in  order  that  a  new 


INTRODUCTION  11 

character  may  be  produced  there  must  be  a  modification  of  the 
germ  plasm.  The  real  question,  then,  is  what  causes  germinal 
change?  In  considering  this  question  we  must  keep  in  mind  the 
possibility  that  agencies  which  are  of  little  importance  from  the 
standpoint  of  the  plant  breeder  may  be  of  profound  importance 
in  evolution. 

DeVries'  Mutation  Theory. — The  more  recent  theory  of  evo- 
lution developed  by  DeVries  attacks  the  question  of  the  sort 
of  variations  which  furnish  the  basis  for  evolution.  DeVries 
gives  only  slight  value  to  the  small  continouus  variations  and 
advances  the  hypothesis  that  large  variations  are  of  primary 
value.  He  believes  in  periods  of  mutation  when  from  some  un- 
known cause  a  species  is  producing  many  new  forms,  and  other 
periods  when  stability  of  the  species  is  the  rule.  DeVries  recog- 
nized three  sorts  of  mutations;  (1)  progressive,  when  an  entirely 
new  character  appears ;  (2)  degressive,  the  appearance  of  a  par- 
tially latent  or  hidden  character;  and  (3)  retrogressive,  when  an 
active  character  becomes  latent.  The  cause,  or  causes,  of  these 
sudden  changes  was  not  known.  Mutations  are  frequently  not 
large  but  small.  All  sudden  heritable  changes  which  cannot  be 
explained  by  the  laws  of  segregation  and  recombination  are 
called  mutations. 

The  Pure -line  Theory. — The  studies  of  Johannsen  are  of  par- 
ticular value  from  the  standpoint  of  the  plant  breeder.  He 
worked  with  self -fertilized  crops  and  found  that  while  the  progeny 
of  a  single  self-fertilized  plant  varied  quite  widely,  these  varia- 
tions were  not  inherited.  From  single  commercial  varieties  he 
found  it  possible  to  isolate  numerous  lines  which  in  their  means 
differed  slightly  from  each  other  and  which  bred  true.  Johannsen 
considered  a  pure  line  to  be  the  progeny  of  one  or  more  self- 
fertilizations  from  a  single  homozygous  ancestor.  Selection 
within  such  a  pure  line  was  of  no  practical  value.  Numerous 
investigations  with  self -fertilized  crops  have  been  made  and  corrob- 
orate the  results  of  Johannsen.  Isolated  cases  of  mutations  in 
these  pure  lines  have  been  reported,  and  while  these  are  of  much 
scientific  interest  they  occur  far  too  infrequently  to  be  used  as  a 
basis  for  a  system  of  breeding. 

Johannsen  '&  pure-line  theory  has  been  extended  to  cover 
clonal  or  asexual  propagation  in  both  plants  and  animals.  At  its 
proper  place  evidence  will  be  given  to  show  that  in  heterozygous 
organisms  which  are  asexually  propagated  there  sometimes  occur 


12  BREEDING  CROP  PLANTS 

bud  sports  or  somatic  mutations  each  of  which  may  form  the 
basis  for  a  new  race.  Such  bud  sports  in  some  plants  apparently 
occur  frequently  enough  to  be  of  economic  importance. 

Mendel's  Law  of  Heredity. — Mendel's  experiments,  pub- 
lished in  1866,  remained  unnoticed  until  the  facts  were  redis- 
covered in  1900  by  De  Vries,  by  Correns,  and  by  Tschermak. 
This  law  furnished  the  starting  point  from  which  the  modern 
study  of  genetics  has  developed.  Many  students  will  have  taken 
a  course  in  genetics  before  studying  plant  breeding.  For  such 
students  it  is  sufficient  here  briefly  to  review  Mendel 's  law  in  its 
application  to  crop  improvement. 

Mendel's  law  can  best  be  understood  in  relation  to  cytology.  It 
is  well  known  that  the  chromosomes  are  the  bearers  of  the  herit- 
able factors.  The  number  of  chromosomes  for  each  species 
is  constant  and  the  form  and  individuality  is  characteristic. 
Each  chromosome  is  supposed  to  be  composed  of  chromomeres 
and  each  chromomere  may  be  the  seat  of  a  particular  inherit- 
able factor.  According  to  Morgan 's  hypothesis,  the  factors  are 
located  in  particular  regions  of  the  chromosome.  The  chromo- 
somes are  considered  to  be  in  pairs  and  the  two  parts  of  each  pair 
are  in  such  a  relation  to  each  other  that  at  reduction  division, 
i.e.,  at  the  formation  of  gametes,  the  parts  of  each  pair  separate 
and  the  gamete  contains  only  half  as  much  chromatin  as  the 
somatic  cell.  The  gamete  then  contains  one  member  of  each 
chromosome  pair.  Exceptions  sometimes  occur  to  the  above 
rule  when  unusual  cytologic  divisions  take  place. 

A  rather  recent  development  of  genetics  is  of  primary  impor- 
tance. At  some  time  in  preparation  for  reduction  division  there 
is  a  doubling  of  the  spireme.  Morgan  supposes  that  at  this  time 
homologous  parts  of  chromosome  pairs  lie  next  to  each  other. 
These  spireme  threads  wind  about  each  other  and  in  some  cases 
breaks  occur.  It  is  then  supposed  that  the  chromosomes  may 
reunite  in  such  a  manner  that  a  new  chromosome  is  formed  which 
contains  parts  of  each  of  the  homologous  chromosomes  that  make 
up  a  pair.  If  factors  are  in  particular  loci  this  would  allow  for  a 
different  combination  of  factors  in  a  chromosome  containing 
parts  of  each  chromosome  pair. 

Most  of  the  previous  investigations  show  that  many  factors 
are  inherited  independently.  This  allows  for  numerous  combina- 
tions when  crosses  are  made.  If  there  is  a  break,  i.e.,  a  cross- 
over or  some  other  means  by  which  factors  which  are  usually 


INTRODUCTION  13 

correlated  may  be  recombined,  a  greater  degree  of  segregation  is 
possible  than  when  factor  correlation  is  absolute. 

In  general  we  may  say  that  the  number  of  groups  of  correlated 
or  partially  linked  factors  is  not  greater  than  the  number  of 
chromosome  pairs.  Whether  the  above  explanation  is  correct, 
partly  so,  or  entirely  wrong,  it  is  a  convenient  theory  with  which 
to  account  for  a  large  body  of  facts.  It  allows  for  classification 
of  facts  in  such  a  way  that  correct  breeding  methods  may  be  used. 

Mendel's  law  may  then  be  summarized  from  the  standpoint 
of  the  plant  breeder  as  follows  : 

1.  Plants  breed  true  for  certain  characters  when  all  factors 
necessary  for  the  development  of  the  character  are  in  a  homozy- 
gous  condition.     There  is  a  relative  stability  of  factors.     Changes 
in  factors  or  " mutations"  are  far  too  infrequent  to  furnish  a  basis 
for  a  system  of  breeding. 

2.  There  is  independent  segregation  of  certain  factors. 

3.  Partial  coupling  of  certain  determiners  sometimes  is  found. 
The  degree  of  linkage  in  transmission  is  quite  constant. 

4.  Perfect  coupling  of  certain  factors  occurs,  i.e.,  constant 
association  of  characters  in  inheritance. 

As  a  possible  exception  to  the  usual  behavior  we  may  mention 
apparent  segregation  in  the  somatic  cells  of  some  hybrids.  In 
some  forms  these  changes  apparently  occur  frequently  enough  to 
be  of  practical  selective  value. 

We  may  summarize  Mendel's  law  in  another  way  by  saying 
that  the  first  generation  cross  between  stable  forms  may  resemble 
one  parent  in  one  character,  the  other  parent  in  another  character 
or  may  be  intermediate  in  the  character  in  question.  All  mem- 
bers1 of  FI  are  of  uniform  habit.  Segregation  occurs  in  F2  and 
"  segregation  of  potential  characters  in  the  germ  cells  of  hybrids 
arid  their  chance  recombination"  (East  and  Hayes,  1911)  may  be 
considered  as  a  general  law.  In  Fs  and  later  generations  some 
forms  breed  true,  others  segregate. 

Homozygous  forms  may  be  obtained  which  contain  the  de- 
sirable characters  of  both  parents.  Such  forms  are  as  stable 
as  races  which  ha\e  been  bred  by  straight  selection. 

Hybridization  as  a  Means  of  Producing  Variations. — A  quite 
recent  explanation  for  the  cause  of  germinal  variation  and 
therefore  the  main  cause  of  evolution  is  that  of  Lotsy  (1916), 

1  The  meaning  of  FI,  Ft,  etc.,  and  other  genetic  terms  not  explained  in  the 
text  is  given  in  the  glossary. 


14  BREEDING  CROP  PLANTS 

who  gives  to  hybridization  the  major  role  in  the  production  of 
variations.  Some  serious  criticisms  have  been  made  of  this 
hypothesis  as  an  explanation  of  evolution.  With  the  higher 
plants,  however,  natural  crossing  has  doubtless  played  an  im- 
portant evolutionary  role.  From  the  standpoint  of  the  plant 
breeder  crossing  is  of  much  importance  and  it  is  the  only  generally 
known  means  of  producing  variations  of  selection  value  that  is 
available  as  a  practical  method.  In  cross-fertilized  species  crosses 
naturally  occur  followed  by  segregation,  and  recombination 
follows.  Selection  isolates  desirable  genotypes. 

THE  VALUE  OF  CROP  IMPROVEMENT  IN  RELATION  TO  A  MORE 
EFFICIENT  AGRICULTURE 

Maximum  yields  of  crops  can  be  obtained  only  when  all 
factors  relating  to  the  various  phases  of  crop  production  are 
favorable.  The  physical  and  chemical  characteristics  of  the  soil, 
correct  time  and  rates  of  planting,  and  crop  rotation  must  be 
considered.  Recent  studies  have  shown  that  there  are  marked 
differences  in  the  effect  of  different  crops  upon  those  that  follow 
them  in  the  rotation.  Of  utmost  importance  is  the  necessity 
that  the  crop  be  adapted  to  the  soil  and  climatic  conditions 
in  which  it  is  to  be  grown  and  that  profitable  returns  be  obtained 
on  the  basis  of  the  cost  of  production. 

After  careful  consideration  of  those  factors  which  go  to  make 
up  the  home  of  the  plant  we  turn  our  attention  to  the  seed.  The 
fact  that  there  are  remarkable  differences  in  final  yields  from 
different  varieties  of  the  same  crop  is  commonly  known.  We  are 
as  yet  only  on  the  threshold  of  the  possibilities  of  crop  improve- 
ment. Careful  methods  of  seed  inspection,  registration,  and 
treatment  to  control  diseases  are  necessary  to  the  greatest 
return  from  crop  breeding.  Education  of  the  farmer  will  do 
much  to  overcome  the  evils  of  exploitation  by  the  unscrupulous 
seed  dealer  or  promoter  who  is  anxious  only  to  sell  and  make 
a  profit  on  his  seed. 

The  business  of  growing  carefully  bred  seeds  is  one  that  needs 
an  appreciation  of  these  and  other  factors  in  seed  production. 
No  great  amount  of  special  training  is  needed  to  carry  on  this 
work.  To  the  careful  worker  who  is  willing  to  build  up  a  reputa- 
tion by  actual  merit  of  his  seeds,  the  business  of  seed  production 
will  prove  a  lucrative  one. 


INTRODUCTION  15 

The  production  of  improved  forms  by  breeding  is  a  line  of  work 
which  demands  special  training.  This  can  be  obtained  only  from 
a  study  of  the  underlying  principles  of  genetics.  Nearly  all  of  our 
land  grant  colleges  and  experiment  stations,  as  well  as  some 
private  seed  firms,  are  carrying  on  studies  in  plant  breeding. 
Although  these  studies  are  yet  in  their  infancy,  results  of  much 
value  are  being  obtained.  By  means  of  accurate  field  experi- 
ments carried  on  at  research  stations  and  with  farmer  codperators, 
the  experiment  stations  and  the  federal  Department  of  Agricul- 
ture are  enabled  to  give  accurate  information  regarding  the  better 
varieties  to  grow.  In  the  past  these  studies  have  not  always 
been  made  with  a  correct  appreciation  of  the  necessary  technic. 

It  is  the  purpose  of  this  book  to  outline  methods  of  breeding 
in  relation  to  the  underlying  principles  involved,  and  to  present 
what  are  coming  to  be  recognized  as  proper  field  methods  of 
carrying  on  these  studies.  Because  the  subject  is  a  compara- 
tively recent  one,  new  methods  of  work  are  being  constantly 
found.  It  is  therefore  necessary  to  present  different  viewpoints 
in  order  that  the  prospective  breeder  may  learn  why  certain  prac- 
tices are  giving  the  better  results. 


CHAPTER  II 
PLANT  GENETICS1 

Since  the  rediscovery  of  Mendel's  law  in  1900  there  has  been 
an  intensive  study  of  the  laws  of  inheritance  through  experimental 
breeding  and  other  means.  This  has  resulted  in  the  develop- 
ment of  a  new  biological  science  which  is  called  Genetics.  A 
knowledge  of  the  principles  of  this  science  is  a  necessity  if  the 
student  of  crop  breeding  is  to  pursue  his  work  in  the  most  logical 
manner.  The  writers,  therefore,  believe  that  a  study  of  genetics 
should  precede  plant  breeding.  There  are,  however,  many 
people  interested  in  crop  improvement  who  have  not  had  an 
opportunity  to  pursue  an  intensive  study  of  genetics.  For  this 
reason  it  seems  advisable  to  present  genetic  principles  in  some 
detail. 

Methods  of  Studying  Inheritance  of  Characters. — The  charac- 
ters of  a  plant  are  those  qualities  which  serve  to  identify  it. 
They  are  the  means  whereby  one  variety  is  differentiated  from 
another.  The  production  of  a  variety  with  only  desirable 
characters  is  the  main  aim  of  the  breeder.  It  is  a  commonly 
accepted  fact  among  geneticists  that  Mendel's  law  may  be 
used  to  explain  the  inheritance  of  nearly  all  plant  and  animal 
characters.  The  character  is  considered  to  be  the  end  result  of 
the  interaction  of  certain  inherited  factors  which  are  located 
in  the  germ  cells;  these  factors  under  favorable  environmental 
conditions  cause  the  production  of  the  character.  Thus  environ- 
ment and  heredity  both  play  important  roles  in  character 
development.  The  laws  of  inheritance  have  been  developed 
mainly  by  controlled  crosses  between  parents  of  known  in- 
heritance. By  correlating  the  facts  of  character  transmission 
from  parent  to  offspring  with  known  facts  of  cytology,  an 
idea  of  the  mechanism  of  heredity  has  been  obtained.  Before 
presenting  a  description  of  the  factorial  scheme  which  has  been 

1  In  preparing  this  chapter  other  works  on  genetics  have  been  freely  used. 
BABCOCK  and  CLAUSEN  (1918)  and  EAST  and  JONES  (1919)  have  been  par- 
ticularly helpful. 

16 


PLANT  GENETICS 


17 


developed  to  explain  Mendelian  heredity,  it  will  be  necessary  to 
give  some  of  the  main  facts  of  reproduction  in  plants. 

The  Mode  of  Sexual  Reproduction  in  Flowering  Plants. — 
Nearly  all  higher  plants  produce  seeds  as  the  result  of  the  union 
of  sexual  cells  or  gametes.  Each  body  cell  which  is  capable  of 
further  division  contains  a  nucleus  in  which  the  chromatin  is 
located.  This  chromatin,  which  is  composed  of  a  definite  number 
of  parts  or  chromosomes,  gains  its  name  from  the  fact  that  it 
takes  a  dark  stain  with  certain  reagents  when  other  parts  of  the 


Male 


Archesporial  or  megaspore 
mother  cell 

eduction  division 

Period 

Nucleus  with  of 

chromosomes    divisio 


Embryo  sac 


Archesporial,  cell 


Synergids 
Egg  cell  or  female  gamete/ 

Embryo  sac 

Polar  or  endosperm  nuclei 
Antipodal   cells  of 
nutritional  value 


2-celled  stage 
or  diad 


4  -celled  stage 
or  tetrad 


Pollen 
grain 


nucleus 
Generative 
nucleus 


FIG.  3.  —  Diagram  to  illustrate  production  of  male  and  female  gametes.  A, 
In  some  forms  as  Lilium  this  megaspore  mother  cell  functions  directly  as  the 
embryo  sac".  Reduction  division  in  Lilium  occurs  at  the  first  nuclear  division 
within  the  embryo  sac. 

cell  are  unstained.  In  the  soma  or  body  of  the  plant  the  nucleus 
of  each  cell  contains  a  definite  number  of  chromosomes,  half  of 
which  were  obtained  from  the  male  sexual  cell  and  half  from  the 
egg  cell.  Each  new  body  cell  results  from  the  longitudinal 
division  of  the  chromosomes  of  a  preceding  body  cell.  Thus 
all  of  the  somatic  cells  of  a  plant  contain  the  same  number  of 
chromosomes. 

Preparatory  to  the  formation  of  the  germ  cells  or  gametes, 
the  chromosomes  assume  a  paired  condition,  one  member  of  each 


18 


BREEDING  CROP  PLANTS 


pair  being  obtained  from  the  male  parent  and  the  other  from  the 
female  parent.  At  the  formation  of  the  sexual  cells,  or  at  reduc- 
tion division,  one  member  of  each  pair  of  chromosomes  passes  to 
each  daughter  cell  thus  reducing  the  chromosome  number  to  half 
that  in  the  body  cells.  Following  this  reduction  division  there  is 
an  equating  division  whereby  each  chromosome  is  qualitatively 


FIG.  4. — Anther  and  pollen  of  the  lily. 

A,  Mature  anther,  showing  the  four  locules,  or  chambers,  containing  pollen  grains:  the 
anther  opens  lengthwise  on  both  sides  along  the  lines  of  cells  shown  at  a;;  B,  stages  in  the 
formation  of  pollen  grains  in  a  group  of  four  (tetrad)  within  the  pollen  mother  cell;  C, 
mature  pollen  grain  with  early  stages  in  the  development  of  the  male  gametophyte;  t,  tube 
nucleus;  g,  generative  nucleus.  (After  Bergen  and  Davis.) 

equally  divided.     This  results  in  the  formation  of  the  male  or 
female  sexual  cells  or  gametes  as  they  are  called  (see  Fig.  3). 

The  male  sexual  cells  are  produced  in  the  anthers  and  are 
carried  in  the  pollen  grains.     A  mature  pollen  grain  contains  two 
nuclei,  a  tube  nucleus  and  a  generative  nucleus  (see  Fig.  4). 
After    the    pollen    grain   falls   on   the   pistil   the   tube   cell 


PLANT  GENETICS 


19 


elongates,  forming  a  pollen  tube  which  passes  down  the  style. 
This  tube  grows  through  the  tissue  of  the  pistil  and  reaches  the 
embryo  sac.  The  generative  nucleus  passes  into  the  pollen  tube 
and  divides,  forming  two  nuclei  which  are  the  male  gametes. 
The  pollen  tube  grows  through  the  tissues  of  the  pistil  until  it 
reaches  the  embryo  sac,  and  the  tip  of  the  tube  breaks  after  it 
penetrates  the  wall  of  the  embryo  sac.  In  fertilization  one  of 


FIG.  5. — Longitudinal  section  of  the  lower  portion  of  the  embryo  sac  of 
maize  at  the  time  of  fertilization;  pn,  polar  nuclei  fusing;  sn,  sperm  nucleus 
fusing  with  a  polar  nucleus,  pn;  e,  egg;  sn,  sperm  nucleus  in  the  egg;  pt,  pollen 
tube;  syn,  synergid;  v,  vacuole.  (After  Miller.) 

these  gametes  of  the  pollen  tube  unites  with  the  egg  cell  to  form 
the  embryo  of  the  seed  and  the  other  unites  with  two  polar  nuclei 
to  form  the  endosperm  (see  Fig.  5). 

If  we  represent  the  chromosome  number  of  each  body  cell  by 
2x,  each  gamete  would  be  represented  by  x,  the  embryo  formed 
by  the  union  of  the  generative  cell  with  the  egg  cell  would  be 
2x  and  the  endosperm  tissue  3x. 


20  BREEDING  CROP  PLANTS 

The  Inheritance  Factors. — The  inherited  character  is  con- 
sidered to  be  the  result  of  certain  definite  factors  which  are 
located  in  the  chromosome.  Moreover,  a  factor  is  considered  to 
be  located  at  a  certain  definite  place  in  the  chromosome.  After 
the  rediscovery  of  Mendel's  law  in  1900,  numerous  crosses  were 
studied.  In  many  cases  the  inheritance  of  each  differential 
character  in  which  the  parents  differed  was  easily  explained  by 
the  hypothesis  that  one  parent  contained  a  genetic  factor  for  the 
development  of  the  character  and  that  the  other  parent  lacked 
this  factor.  This  led  to  the  erroneous  conception  that  many 
characters  were  dependent  on  a  single  factor  for  their  develop- 
ment. That  this  is  not  so  may  be  easily  seen  if  one  considers 
that  each  character  is  a  part  of  the  physiological  complex  which 
goes  to  make  up  an  organism.  Thus,  many  genetic  factors  play 
a  part  in  the  development  of  the  character.  When  a  cross  shows 
that  two  parents  differ  by  only  a  single  factor  this  does  not  mean 
that  only  a  single  factor  is  necessary  for  the  development  of  a 
character.  It  does  mean,  however,  that  a  single  factor  of  in- 
heritance may  cause  a  profound  change  in  the  expression  of  the 
character  of  an  organism.  Some  crosses  show  that  many  factors 
play  a  part  in  the  development  of  a  single  character.  The  present 
view  is  that  a  character  is  usually  the  result  of  the  interaction 
of  several  factors.  When  a  plant  breeds  true  for  a  particular 
character  each  gamete  produced  contains  all  factors  necessary 
for  the  development  of  the  character.  Before  considering  the 
results  of  certain  crosses  it.  will  be  desirable  to  review  briefly 
the  subject  of  variation. 

Variability  of  Characters. — It  is  commonly  recognized  that 
no  two  plants  or  animals  are  exactly  alike.  These  differences  are 
called  variations.  Various  means  of  classifying  variations  have 
been  used.  From  the  standpoint  of  the  plant  breeder  variations 
are  of  two  kinds:  (1)  non-heritable,  (2)  heritable. 

Non-heritable  variations  are  those  which  are  solely  due  to 
some  difference  or  differences  in  the  environmental  conditions 
under  which  the  plants  develop,  while  heritable  variations  are  due 
to  some  difference  or  differences  in  the  hereditary  characters 
of  the  organisms. 

Several  illustrations  may  help  to  make  clear  what  is  meant 
by  non-heritable  variations.  Baur  (1914)  cites  races  of  Primula 
sinensis  which  under  normal  conditions  breed  constanlly  true 
for  red  and  white  flowers  respectively.  If  the  red  race  is  placed 


PLANT  GENETICS  21 

in  partial  shade  in  the  greenhouse  under  temperatures  of  30°  to 
35°C.  only  white  flowers  are  produced.  If  those  same  plants 
are  brought  into  another  greenhouse  with  temperatures  of  15°  to 
20°C.  the  flowers  which  then  develop  are  the  normal  red  color. 
It  is  pointed  out  that  what  this  red  primula  inherits  is  not  a  red 
flower  color  but  the  ability  to  produce  a  certain  flower  color 
under  certain  conditions  of  the  environment.  Non-inherited 
variations  have  no  value  as  a  means  of  producing  new  varieties  or 
strains.  Such  variations  are,  however,  of  importance  to  the 
breeder.  For  example,  a  small  shriveled  seed  of  wheat  has  the 
same  inherited  characters  as  a  large,  plump  seed  of  the  same 
pure  line,  nevertheless,  the  seedling  produced  by  the  shriveled 
seed  may  get  an  unfavorable  start.  Familiar  examples  of  non- 
heritable  variations  are  differences  in  height  of  plants,  within  a 
variety,  which  are  dependent  on  differences  in  food  supply, 
moisture,  or  sunlight. 

Inherited  variations  may  be  placed  in  two  classes:  (1)  muta- 
tions, (2)  new  combinations. 

Mutations  are  due  to  a  sudden  change  in  the  hereditary  factors 
of  an  organism,  or  to  the  loss  of  a  genetic  factor.  In  some  cases 
mutations  result  from  abnormal  chromosome  behavior  during 
the  process  of  cell  division.  Before  we  can  discuss  profitably 
the  reason  why  mutations  occur  it  will  be  necessary  to  know 
much  more  about  the  nature  of  hereditary  factors  than  we  now 
do.  Mutations  are  sometimes  of  much  value  to  the  breeder. 
Examples  of  mutations  of  economic  importance  will  be  found 
under  a  discussion  of  the  breeding  of  various  crops.  When  a 
desirable  mutation  occurs  it  can  be  utilized  as  a  means  of 
producing  a  new  race.  As  there  is  no  known  means  of  artificially 
inducing  mutations,  the  breeder  can  not  depend  on  them  as  a 
means  of  producing  improved  varieties. 

New  combinations  result  from  crossing  varieties  which  contain 
different  hereditary  factors.  The  first  generation  of  a  cross 
between  homozygous  parents  which  differ  in  a  certain  character 
may  resemble  either  the  one  or  the  other  parent  or  may  be  inter- 
mediated, but  all  FI  plants  will  be  of  like  habit.  F2  plants,  how- 
ever, are  of  different  kinds,  due  to  the  segregation  of  hereditary 
factors  in  the  germ  cells  of  the  FI  plants.  New  combinations  of 
factors  may  occur  and  thus  new  individuals  may  be  produced 
which  have  some  of  the  characters  of  the  one  parent  combined 
with  some  characters  of  the  other.  In  some  cases  characters 


22 


BREEDING  CROP  PLANTS 


which  are  not  present  in  either  parent,  appear.  These  may 
result  from  the  interaction  of  two  or  more  factors  all  of  which 
are  necessary  for  the  production  of  the  character  and  part 
of  which  were  contained  in  one  parent  and  part  in  the  other 
parent.  These  facts  may  be  illustrated  by  the  results  of  certain 
crosses. 

A  Cross  in  Which  the  Parents  Differ  by  a  Single  Factor. — 
Sweet  corn  when  mature  bears  wrinkled  seed  while  flint  corn 
produces  smooth  seeds  filled  with  starch  grains.  If  sweet  corn 


FIG.  6. — Inheritance  of  starchy  and  sweet  endosperm  in  maize.  A,  Ear 
of  sweet  corn  with  wrinkled  seeds;  C,  ear  of  flint  corn  with  starchy  seeds;  B, 
immediate  result  of  pollinating  ear  of  starchy  parent  with  pollen  from  sweet 
parent;  D,  produced  by  self-fertilizing  an  ear  of  an  Fi  plant  of  cross  between 
sweet  and  starchy  parent.  Note  the  segregation  into  sweet  and  starchy  seeds; 
E.  An  ear  produced  by  planting  wrinkled  seeds  of  D;  F,  G,  H,  ears  produced  by 
planting  starchy  seeds  of  D.  Note  that  one  out  of  every  three  ears  is  pure  for 
the  starchy  character.  (After  Babcock  and  Clausen.) 

is  pollinated  with  pollen  from  a  flint  variety  the  resultant  seed 
is  starchy.  There  is  an  immediate  effect  due  to  double  fertili- 
zation in  which  the  endosperm  results  from  the  union  of  the 
polar  nuclei  with  one  of  the  gametes  of  the  pollen  grain.  If  the 
crossed  seeds  are  planted  and  the  resultant  plants  self-fertilized, 
the  ears  produced  will  contain  starchy  and  sweet  seeds  in  a  3 : 1 
ratio.  The  facts  may  be  presented  by  the  use  of  the  factor 
hypothesis.  One  of  the  chromosome  pairs  contains  the  factors 
for  either  the  starchy  or  the  sweet  condition.  Let  S  represent 
the  sweet  factor,  F  the  starchy  factor.  In  the  following  diagram 
only  one  of  the  chromosome  pairs,  which  contains  the  starchy 
and  sweet  factors,  will  be  shown. 


PLANT  GENETICS 


23 


P|     I  F  |    Somatic  cell 

of  flint  parent 


s   |     |  S  I  Somatic  cell 

of  sweet  parent 


Gametes  of  parents 


Ofstaicby  appearance 


Will  breed 
true 


seed  of  starch)   habit .    The  factor 
foi  starch  development,  F,  produces 
a  complele  dominance  over  the  sweet 
factor,  S 


Male  reproductive  cells 


Female  reproductive  cells 


li        Is!  \\  F 


seeds 


wrinkled  seeds 


Will  segregate 

DIAGRAM   1 


Will  breed  true 


Inheritance   of  Two   Independently   Inherited  Characters. — 

Crosses   between   varieties   which   differ   in   two  independently 

inherited  characters  may  next  be  illustrated.  The  parental 
forms  in  the  case  of  each  differential  character  will  be  considered 
to  differ  in  only  a  single  inherited  factor. 

PARENTS                                           CHARACTERS  GAMETES 

White  Fife  wheat               Awnless  spike,  white  seed  AW 

Preston                                Bearded  spike,  red  seed  BR 


24 


BREEDING  CROP  PLANTS 


There  is  a  dominance  in  FI  of  the  red  seed  color  (brownish  red 
pigment  in  one  of  the  bran  layers)  over  the  white  and  a  partial 
dominance  of  the  awnless  over  the  bearded  condition.  The  FI 
plants  will  therefore,  have  red  seeds  and  a  slight  extension  of 
the  awns  near  the  top  of  the  spike. 

The  inherited  factors  may  be  considered  to  be  R  for  red  seed, 
W  for  white  seed,  B  for  bearded,  and  A  for  awnless.  W  and  R 
are  considered  to  be  located  in  homologous  loci  of  one  pair  of 
chromosomes  and  B  and  A  in  homologous  loci  of  another  pair  of 
chromosomes.  The  FI  plants  may  then  be  considered  as  ABWR. 
The  gametes  of  these  FI  plants  may  contain  either  A  or  B  in 
combination  with  either  W  or  R.  The  different  combinations 
are  supposed  to  occur  in  equal  frequency. 

Wheat  (Triticum  vulgare)  has  eight  pairs  of  chromosomes. 
The  factors  for  bearded  or  awnless  spike  and  for  color  of  seed  are 
independently  inherited.  Therefore  they  may  be  considered  to 
be  located  in  separate  chromosome  pairs.  In  the  diagram  only 
two  chromosome  pairs  are  shown. 


SOMT/C  Ceu 


PAKEHTAL  GAMETES 


F,  ZYOOTC 


Ft  GAMETES 


PAREHT  }. 
dwn/ess  spiff? 
White  seed 


Bearded  spike 
Red  seed 


DIAGRAM  2 


The  F2  plants  obtained  by  the  self-fertilization  of  FI  crosses 
will  then  be  the  result  of  all  possible  combinations  of  the  gametes. 
The  combination  will  be  illustrated  by  the  Punnet  square. 


PLANT  GENETICS 


25 


AR 


AW 


BR 


Female 


gametes 


AR 


AW 


BR 


BW 


AR 

AW 

BR 

BW 

AR 

AR 

AR 

AR 

AR 

AW 

BR 

BW 

AW 

AW 

AW 

AW 

AR 

AW 

BR 

BW 

BR 

BR 

BR 

BR 

AR 

AW 

BR           BW 

BW           BW 

BW           BW 

BW        Male  gametes 


Collecting  the  various  combinations  we  obtain: 

^'2  PLANTS  Ft  BREEDING  HABIT 

1  AARR     Awnless,  red  seed.         Will  breed  true  for  awnless  spike  and  red 


2  ABRR  Int.  awns,  red  seed.  Will  segregate  for  spike  character  and 

breed  true  for  red  seed. 

2  AARW  Awnless,  red  seed.  Will  breed  true  for  awnless  spike  and 

segregate  for  seed  color. 

4  ABRW  Int.  awns,  red  seed.  Will  segregate  for  both  seed  color  and 

spike  habit. 

1  AAWW  Awnless,  white  seed.     Will  breed   true  for  awnless  spike  and 

white  seed. 

2  ABWW   Int.  awns,  red  seed.       Will  segregate  for  spike  habit  and  breed 

true  for  white  seed. 

1  BBRR      Bearded,  red  seed.         Will  breed  true  for  bearded  spike  and 

red  seed. 

2  ABRR     Int.  awns,  red  seed.       Will  segregate  for  spike  habit  and  breed 

true  for  red  seed. 

1  BBW W    Bearded,  white  seed.     Will  breed  true  for  bearded  spike  and 

white  seed. 

Several  Factors  Necessary  for  the  Production  of  a  Character. — 

In  many  cases  several  factors  are  involved  in  the  production  of  a 
single  character.  Thus  the  purple  aleurone  color  found  in  Black 
Mexican  sweet  corn  is  dependent  on  the  interaction  of  the  factors 
R,  C,  A,  and  Pr  (see  Chapter  XII,  Maize  Breeding).  C  and  A  are 
basic  factors  both  of  which  must  be  present  for  the  development 
of  color.  When  R,  C  and  A  are  present  the  color  in  the  aleurone 
layer  is  red.  Let  us  study  a  cross  between  Black  Mexican  which 
is  homozygous  for  purple  aleurone  color  and  a  white  sweet  which 
is  homozygous  for  factors  R  and  A  but  which  lacks  the  factors  C 


26  BREEDING  CROP  PLANTS 

and  Pr.     The  lack  of  the  factor  may  be  represented  by  a  small 
letter. 

PARENTS  APPEARANCE  GAMETES  Fi  CROES 

Black  Mexican Purple  color  PrRAC         PrprRRAACc 

White  Sweet White  color  prRAc 

As  the  FI  seeds  contain  all  factors  necessary  for  the  production 
of  purple  color  in  the  aleurone  layer,  they  will  be  purple.  In  later 
generations  the  factors  R  and  A  may  be  considered  to  be  present  in 
each  gamete,  as  both  parents  were  homozygous  for  these  characters. 
The  gametes  of  the  Fl  plants  will,  therefore,  be  PrRAC,  prRAC, 
PrRAc,  and  prRAc.  By  the  Punnet  square  method  as  illustrated 
in  the  previous  topic,  the  student  may  determine  the  possible  F2 
combinations.  These  will  be  found  to  occur  in  the  following 
proportions : 

COMBINATIONS  APPEARANCE 

1  PrPrRRAACC 

2  PrprRRAACC 

2  PrPrRRAACc  9  Purple  aleurone 


4  PrprRRAACc 

1  prprRRAACC 

2  prprRRAACc 

1  PrPrRRAAcc 

2  PrprRRAAcc 
1  prprRRAAcc 


3  Red  aleurone 


4  White  aleurone 


Linkage  of  Characters  in  Inheritance.— Morgan  and  his  co- 
workers  have  made  an  intensive  study  of  the  inheritance  of 
characters  in  the  fruit  fly,  Drosophila  melanog aster.  Over  300 
inherited  factors  have  been  studied.  If  these  inherited  factors 
are  located  in  the  chromosomes  and  as  there  are  only  four  pairs 
of  chromosomes  in  the  fruit  fly,  it  would  seem  that  certain  fac- 
tors should  be  linked  together  in  their  transmission.  That  this 
is  the  case  has  been  clearly  proved  by  the  result  of  many  studies. 

What  frequently  happens  is  that  factors  which  tend  to  be 
present  in  the  same  chromosome  or  gamete,  sometimes  change 
their  linkage  relations.  That  breaks  do  occur  in  the  chromosomes 
seems  evident  from  careful  cytological  studies.  Preparatory  to 
reduction  division  the  two  chromosomes  which  make  up  each 
each  pair  lie  side  by  side.  There  is  often  a  twisting  of  these 
chromosomes  about  each  other  and  in  some  cases  breaks  occur 
and  the  parts  are  joined  in  a  new  relation  which  causes  a  modi- 
fication of  the  linkage  relation. 


PLANT  GENETICS 


27 


The  diagram  illustrates  a  change  in  linkage  relations  due  to  a 
cross-over.  C  and  W  are  located  in  the  same  chromosome  of  one 
parent  and  c  and  w  in  homologous  loci  of  a  similar  chromosome 
of  the  other  parent.  If  there  was  perfect  linkage  the  only 
gametes  produced  would  be  CW  and  cw.  Owing  to  a  cross-over, 
however,  Cw  and  cW  are  also  obtained  although  less  frequently 


FIG.  7. — Diagrammatic  representation  of  crossing-over  and  results.  At  the 
left,  the  two  original  chromosomes.  In  the  middle,  the  twisted  condition  of 
the  chromosomes  in  synapsis  and  their  subsequent  separation.  At  the  right, 
the  four  types  of  chromosomes  which  result.  (After  Babcock  and  Clausen.} 

than   the    combinations   CW   and   cw.     The   following   outline 
expresses  the  result  on  a  percentage  basis: 

CW  38.7  percent.;  cw  38.7  per  cent.     cW  11.3  per  cent.;  Cw  11.3  percent. 


non-cross-over  gametes 


cross-over  gametes 


Accepting  the  view  that  factors  are  located  in  particular  places  in 
the  chromosome,  the  value  of  the  cross-over  hypothesis  in  explain- 
ing degrees  of  factor  linkage  becomes  apparent.  If  certain  com- 
binations of  factors  occur  with  less  frequency  than  others,  this 
means  that  the  breeder  must  grow  a  much  larger  population  in  the 
segregating  generations  in  order  to  obtain  the  combination  desired 
than  when  the  factors  are  independently  inherited. 

Inheritance  of  Quantitative  Characters. — Many  of  the  impor- 
tant characters  of  economic  plants  are  size  or  quantitative 
characters,  such  as  height  of  plants,  size  of  seed,  or  relative  date 
of  maturity.  It  was  at  first  thought  that  these  characters  did 
not  follow  Mendel's  law.  The  discovery  that  color  characters 
were  frequently  due  to  the  interaction  of  several  inherited  factors 
led  to  the  explanation  of  the  inheritance  of  size  characters  by 
similar  means.  Numerous  controlled  crosses  have  been  studied. 


28  BREEDING  CROP  PLANTS 

The  general  nature  of  the  results  in  this  field  may  be  illustrated 
by  a  cross  between  barley  varieties  which  differ  in  the  average 
length  of  mternodes  of  the  rachis  (see  Table  I). 

In  this  cross  between  Hanna  and  Zeocriton,  lax  and  dense 
varieties  respectively,  the  F2  ranged  from  above  the  modal  class 
of  Hanna  to  the  modal  class  of  Zeocriton  even  though  only  141 
individuals  were  studied.  The  calculated  coefficient  of  varia- 
bility for  the  F2  was  three  or  four  times  greater  than  for  the 
parental  varieties.  Several  small  F3  families  were  grown  from 
Fz  plants  representing  different  densities.  By  examining  the 
table  one  will  note  that  some  F3  lines  bred  comparatively  true, 


FIG.  8. — Average  spikes  of  the  Zeocriton  (left),  Hanna  (right)  and  four  homo- 
zygous  lines.  Mean  densities  are  as  follows:  Zeocriton,  1.9  mm.;  Hanna  X 
Zeocriton,  448-1,  2.3  mm.;  448-5,  2.9  mm.;  448-11-3,  3.7  mm.;  448-16,  4.3  mm.; 
Hanna,  4.6  mm. 

the  ranges  for  density  being  no  greater  than  for  the  parental 
lines  and  the  coefficients  of  variability  also  being  low.  Other 
7'!3  lines  were  as  variable  as  the  F2  generation  while  still  others 
were  more  variable  than  the  parents  but  less  variable  than  the  F2. 
Several  Fs  lines,  which  appeared  homozygous,  were  tested  in 
F4  and  some  of  these  on  the  basis  of  the  more  extensive  test  again, 
gave  evidence  of  homozygosity.  The  general  nature  of  the 
results  is  illustrated  in  Fig.  9.  These  results  show  that  homo- 
zygous lines  for  density  may  be  obtained  in  F3  and  F4,  and  that 
in  this  cross  homozygous  lines  were  obtained  which  approached 
the  densities  of  the  parents  as  well  as  homozygous  lines  with 


PLANT  GENETICS 


29 


>* 

OOOOOrHOOOOOOOOrHQOO 

ooooooooooo 

d 

N                        i-H                        i-l                        W               i-l 

1 

cq,-iT-ii-irHT}<NrHcoc<i<-'COrHi-ico^HTt<'<f 
oooooooooooooooooo 

d  d  d  d  d  d  d  d  d  d  d  d  d  d  d  d  d  d 

§C^rHCqTt<CCC^C<INrHU5 
oooooooooo 

d  d  d  d  d  d  d  d  d  d  d 

-H  +1  -H  +1  +1  -H  +1  +1  +1  +1  +1 

«  rtJ  «  rn  CN'  ra  N  «  N  CN'  c^  N  N  w  N  N  w  ra 

<«•  «  co  co'  co  «  «  ^  ^  «  * 

h 

aS8as3s3ss|8||98S2 

rH                               CO 

9'S 

s 

fr  e 

1-1 

z  e 

* 

O'S 

^       2 

~                               CO    rH 

8'* 

CO 

rHCOO 

9'* 

^gi 

CN                                        CO             rH    00    rH    •* 

a    *•* 

a    r  v 

^J    CO    CO                                                                                                W 

^                                                               °°     J2    bl     "° 

CH 

2    CO    M                    **                                                                     CO            t- 

^            ^^     S2g^ 

•S      0'* 

CO    CO                           i-H                                                                                iH 

ri    CO              IN               CO     rH 

"«      8'8 

CO    0    ^                   «5                                                                 (N            IN    ^ 

rHU5              222              W     -H     JH     N 

I     ••« 

CO     rH                                     Oi                                                                               •*     W                CO     CO 

OOrHOSCOCO-*                      (NrH 

a.  re 

(N                  M           i-l                  00    1-1          t^    >O 

CO    Th    ^   tN    ^    CO                          CO 

1  *'8 

00                  Tj<           OOCO           OM<           »OiO 
i-H                                       CO                      •*                      i-H 

"5   O          (N    00    OS                         CO 
rH    CO             rH    r-l    W 

I    o-. 

t^                  t^iO^rfl           (NOO           OOTH 
0^         ^ 

CO    b-             CO    Oi    CO                               rH 

;  «•« 

^    jH     ^           , 

rHrH             COrH^ 

o    9  s 

i-l             <N              rH    IO                                                 rH 

*'S 

-HO5OOi"CTt<CO           O          J>COt-i 
t^   (N                         (N 

2'2 

IN           --I   00   <N                         lO                  i-l 

0*2 

00<N'*O5'-lt^                      CMC*             l-HlCrH 
1-1     t^                1-1     N     rH                                     N                          CO 

ri 

COCO           lOrHCO                          CO           COCO 

9'T 

lO                           i-i 

£     fi 

22                                  rH^^SoOOrHOCO^C, 

00 

ri 

CO    rf<                                  rH                   IN 

*M"w""""i"* 

», 

oot^oooot^oooooot^oot>. 

i 

O5C35O5O5050i0503O5OSC35O5O50>O5O5a>a> 

O    C3    O    O3    C3    OS    O    Ct)    Oi    Oi    O 

d 

0 



;     •  O     •      •  O     

£ 

S  *  "a  *  *  "oi  

•     •     .     •     •     •     .   ft    . 

> 

S        *-**--*-----oa> 

«                     ••§             -rn'r^^^J^^dUS 

CO              ||         •      1       1       1         •    "^      '• 

I       rHrHrHCOCOCOCOCOCOOO 

c8                    S             ^jOpOOOOQOOpOOOOOOOOOOOOOO 

oooooboooboboboboooooo 

30 


BREEDING  CROP  PLANTS 


intermediate  densities.  The  determination  of  just  how  many 
factors  were  involved  could  not  be  made  without  a  more  extensive 
test.  The  results  can  be  explained  on  a  genetic  basis  by  the 
hypothesis  that  Zeocriton  contains  three  independently  in- 
herited factors  for  density  and  that  Hanna  lacks  these  factors. 
The  added  hypothesis  may  be  made  that  each  factor  in  a  hetero- 
zygous condition  gives  half  as  great  an  effect  as  when  homo- 
zygous.  The  factors  may  be  considered  to  have  a  cumulative 
effect,  two  factors  when  present  in  a  hornozygous  condition 


/ 

\ 

1 

/ 

\ 

/ 

\ 

/ 

\ 

^ 

h 

ocri 
5,-Wl 

\ 

ton 

S^ 

3 

na 

\ 

"-*) 

IT.ib 

X 

""•- 

^ 

/ 

y- 

*2- 

(Ufa 

x 

-5  w 

•S  40 

•a  *u 

$* 

»  20 

i- 

£  o 

/ 

***>, 

\ 

/ 

"-^ 

i 

^ 

/ 

\ 

I 

\ 

i 

! 

1 

/ 
/ 

\ 

/ 

) 

\ 

A 

>^ 

\ 

f 

/ 

448 

1 

A-5\ 

;\ 

448- 

1-3 

-*-  **" 

^448- 

Ik 

x<. 

1.4  1.61.82.0  2.2  2.4  2.6  2.8   3.0  3.2  3.4  3-6  3-8  4.0   4.2  4.4  4.6  4.8  5.0  5.2  5.4  5.6 
la  MM 


FIG.  9.  —  Diagrams  showing  the  densities  of  parental  forms  and  of  the  F 
generation  in  a  cross  between  the  Zeocriton  and  Hanna  barleys  (upper),  of  four 
pure  lines  (middle),  and  of  several  heterozygous  lines  (lower).  (After  Hayes  and 
Harlan,  1920.) 

producing  twice  as  great  an  effect  as  when  a  single  factor  is 
hornozygous.  Other  factors  of  a  smaller  value  are  also  doubtless 
present  which  modify  the  expression  of  the  main  density  factors. 
East  and  Jones  have  summarized  the  results  of  such  controlled 
crosses  and  they  find  a  number  of  general  conditions  fulfilled. 

"1.  When  pure  or  homozygous  races  are  crossed,  the  FI  populations 
are  similar  to  the  parental  races  in  uniformity.  This  conclusion 
devolves  from  observations  that  if  any  particular  factors  AA  and  aa 
are  homozygous  in  the  parental  races,  they  can  only  form  Aa  individuals 
in  the  FI  generation. 

"2.  If  the  parental  races  are  pure,  F2  populations  are  similar,  no 
matter  what  FI  individuals  produce  them,  since  all  variability  in  the 
FI  generation  is  the  result  of  varying  external  conditions. 


PLANT  GENETICS  31 

"3.  The  variability  of  the  F2  populations  produced  from  such  crosses 
should  be  much  greater  than  that  of  the  FI  populations,  and  if  a  sufficient 
number  of  individuals  are  produced  the  grand-parental  types  should  be 
recovered.  The  fulfillment  of  this  condition  comes  about  from  the 
general  laws  of  segregation  of  factors  in  F\  and  their  recombination  in  JFV 

"4.  In  certain  cases  F2  individuals  should  be  produced  showing  a 
greater  or  a  less  extreme  development  of  the  character  complex  than 
either  grandparent.  This  is  merely  the  result  of  recombination  of 
modifiers,  as  was  explained  above. 

"5.  Individuals  of  different  types  from  the  F«  generation  should 
produce  populations  differing  in  type.  The  idea  on  which  this  state- 
ment is  based  is,  of  course,  that  all  F2  individuals  are  not  alike  in  their 
inherited  constitution  and  therefore  must  breed  differently. 

"6.  Individuals  either  of  the  same  or  of  different  types  chosen  from 
the  Fz  generation  should  give  Fz  populations  differing  in  the  amount 
of  their  variability.  This  conclusion  depends  on  the  fact  that  some 
individuals  in  the  F^  generation  will  be  heterozygous  for  many  factors 
and  some  heterozygous  for  only  a  few  factors." 

From  the  standpoint  of  the  student  a  hypothetical  case  may 
be  given  to  show  how  the  factor  hypothesis  may  be  used  to 
explain  the  inheritance  of  quantitative  characters.  Given  two 
barley  varieties  as  follows: 

t  Variety  1,  average  length  of  internode  of  rachis  2.0  mm. 
Variety  2,  average  length  of  internode  of  rachis  3.6  mm. 

Suppose  these  varieties  differ  by  two  separately  inherited  factors, 
A  and  B,  each  when  homozygous  causing  a  lengthening  of  the 
internode  by  0.8  mm.;  when  heterozygous  by  0.4  mm., 

Variety  1     aabb     Gamete   ab    _,    „ 

XT-      •   A       n     A  A  rt-o    r*  A  n  *!    ZygOte   AdBb 

Variety  2  A  ABB  Gamete  AB 
Combinations  in  F2  would  occur  as  follows: 

Fz  PLANTS  Fa  BKEEDING  NATURE 

1  AABB  Would  breed  true  for  length  of  internode  of  3.6  mm. 

2  AaBB  Would  segregate  from  3.6  mm.  to  2.8  mm. 
2  AABb  Would  segregate  from  3.6  mm.  to  2.8  mm. 
4  AaBb  Would  segregate  as  F2. 

1  AAbb       Would  breed  true  for  length  of  internode  of  2.8  mm. 

2  Aabb        Would  segregate  from  2.8  to  2.0  mm. 

1  aaBB       Would  breed  true  for  length  of  internode  of  2.8  mm. 

2  aaBb        Would  segregate  from  2.8  to  2.0  mm. 

1  aabb         Would  breed  true  for  length  of  internode  of  2.0  mm. 

Probably  few  size  characters  are  as  simple  in  their  inheritance  as 
this  illustration.  However,  the  factor  notation  assists  in  gaining 


32  BREEDING  CROP  PLANTS 

a  conception  of  the  mode  of  transmission  of  these  size  characters 
and  there  seems  no  good  reason  for  believing  that  a  different 
mechanism  is  involved  than  in  the  inheritance  of  color  characters. 
Environmental  conditions  probably  play  a  larger  role  in  the 
modification  of  the  appearance  of  size  characters  than  for  color 
characters. 

Stability  of  Inherited  Factors. — That  sudden  changes  in  the 
appearance  of  a  character  are  sometimes  found  is  a  well-known 
fact.  The  causes  of  these  sudden  changes  are  not  so  easily 
determined.  Whether  these  are  more  logically  explained  as  due 
to  changes  in  certain  inherited  factors  or  due  to  a  new  recombina- 
tion of  factors  or  by  other  causes  is  an  unanswered  question. 
The  pure-line  theory  of  Johannsen  was  a  result  of  an  experimental 
attack  on  the  question  of  the  stability  of  a  character.  A  few 
sudden  changes  in  characters  have  been  observed.  Nevertheless, 
plant  characters  of  self-fertilized  crops  exhibit  remarkable 
uniformity.  Many  of  the  inherited  sudden  changes  which 
have  been  noted  are  most  logically  explained  as  the  result  of  a 
natural  cross.  Others  appear  to  be  due  to  a  sudden  change  in 
the  hereditary  factors  of  the  organism  or  to  the  loss  of  a  genetic 
factor. 

The  view  of  factor  stability  which  seems  most  helpful  for  the 
plant  breeder  has  been  clearly  stated  by  East  and  Jones  (1919). 

"For  these  and  other  reasons  which  might  be  given,  could  further 
space  be  devoted  to  the  subject,  we  believe  there  should  be  no  hesitation 
in  identifying  the  hypothetical  factor  unit  with  the  physical  unit  factor 
of  the  germ  cells.  Occasional  changes  in  the  constitution  of  these 
factors,  changes  which  may  have  great  or  small  effects  on  the  characters 
of  the  organism,  do  occur;  but  their  frequency  is  not  such  as  to  make 
necessary  any  change  in  our  theory  of  the  factor  as  a  permanent  entity. 
In  this  conception  biology  is  on  a  par  with  chemistry,  for  the  practical 
usefulness  of  the  conception  of  stability  in  the  atom  is  not  affected  by  the 
•knowledge  that  the  atoms  of  at  least  one  element,  radium,  are  breaking 
down  rapidly  enough  to  make  measurement  of  the  process  possible." 


CHAPTER  III 

THE  MODE  OF  REPRODUCTION  IN  RELATION  TO 
BREEDING 

General  recognition  of  the  stability  of  inherited  factors  has 
served  to  emphasize  the  importance  of  a  knowledge  of  the  mode 
of  reproduction  of  crop  plants.  If  the  crop  in  question  is  nor- 
mally self-fertilized,  and  has  been  bred  carefully,  accidental 
crosses  may  cause  serious  mixtures  in  the  variety  and  thus 
prohibit  its  sale  as  pedigreed  seed.  With  naturally  cross- 
fertilized  plants,  self-fertilization  often  has  a  detrimental  effect. 
A  knowledge  of  the  mode  of  pollination  of  a  crop  is  therefore 
an  absolute  necessity  in  outlining  correct  methods  of  breeding. 

As  with  other  characters,  environmental  conditions  play  an 
important  role.  With  crops  which  are  adapted  for  insect 
pollination  and  yet  which  are  self-fertile,  the  number  and  sort 
of  insects  found  in  the  locality  may  greatly  modify  the  amount  of 
crossing  which  takes  place.  Variations  in  moisture  conditions 
may  determine  the  amount  of  cross-pollination.  The  age  of  the 
plant  also  is  of  importance.  Aside  from  these  there  are  often 
varietal  differences  in  closely  related  forms. 

Plants  may  be  placed  in  four  groups  according  to  their  mode 
of  reproduction.  These  groups,  however,  overlap  because  of 
prevailing  conditions  and  inherent  differences  which  the  plants 
exhibit. 

Group  1. — Naturally  self-pollinated:  Wheat,  oats,  barley, 
peas,  beans,  flax,  tobacco,  tomatoes,  cotton,  sorghums.1 

Group  2. — Often  cross-pollinated:  Maize,  rye,  sugar  beets, 
root  crops,  grasses,  alfalfa,  cucurbits. 

Group  3. — Cross-pollination  obligatory:  (a)  Self -sterile,  red 
clover,  sunflower,  many  fruits;  (b)  Dioecious  plants,  hops, 
hemp,  asparagus,  and  date  palm. 

1  Some  crops,  such  as  sorghum  and  cotton,  cross  in  the  field  frequently. 
As  there  is  no  sharp  line  of  demarkation  between  cross-  and  self -pollinated 
plants  and  as  sorghum  and  cotton  should  apparently  be  handled  by  the 
breeder  in  much  the  same  manner  as  crops  like  barley,  which  is  seldom 
naturally  cross-pollinated,  it  seemed  wiser  to  place  sorghum  and  cotton  in 
the  self-fertilized  group. 

3  33 


34 


BREEDING  CROP  PLANT X 


Group    4. — Vegetatively    propagated:  Potatoes,    sugar    cane, 
many  fruits. 


NATURAL  CROSSING  WITH  SELF -FERTILIZED  PLANTS 

Flower  types  are  adapted  for  various  degrees  of  self-  or  cross- 
fertilization.  This  in  itself  is  a  field  in  which  much  study  might 
be  made.  The  plant  breeder,  however,  is  chiefly  interested  in 
the  final  result. 


FIG*  10. — Natural  hybrids  in  wheat.  1.  From  right  to  left:  Spike  of  a  pure 
variety  produced  from  a  cross  of  Turkey  winter  wheat  and  Wellman's  Fife  spring 
wheat.  This  is  a  bearded  variety  with  smooth  chaff.  The  progeny  of  a  single 
plant  of  this  variety  gave  48  bearded,  smooth  chaffed  plants  and  2  plants  with 
intermediate  (tipped  awns)  and  hairy  chaff.  2.  From  right  to  left:  Preston 
spring  wheat;  an  F\  natural  hybrid  with  intermediate  awns  and  hairy  chaff. 
The  parental  varieties  from  which  these  natural  hybrids  were  obtained  were 
grown  alternately  with  Haynes  Blue  Stem  the  preceding  year. 

Wheat.* — The  individual  florets  of  wheat  and  barley  are 
much  alike.  The  envelope  of  a  floret  of  wheat,  for  example, 
consists  of  the  flowering  glume  or  lemma  and  an  inner  glume  or 
palea.  The  sexual  organs  consist  of  a  pistil  with  a  two-branched, 

1  POPE  has  reviewed  much  of  the  literature  for  cereal  crops.  See  Journ. 
Amer.  Soc.  Agron.,  8:  209-227. 


MODE  OF  REPRODUCTION  IN  RELATION  TO  BREEDING  35 


fl 


feathery  stigma  and  of  three  stamens  with  anthers,  all  of  which 
are  enclosed  by  the  lemma  and  palea.  Opposite  the  base  of  the 
palea  are  two  tiny  sac-like  organs,  lodicules.  The  increase  in  size 
of  these  organs  due  to  water  absorption  causes  the  flower  to  open. 
This  occurs  when  the  stigma  is  receptive  and  at  this  time  the 
elongation  of  the  filaments  causes  the 
anthers  to  protrude  from  the  glumes, 
when  they  promptly  dehisce.  The 
process  of  blooming  is  very  rapid  and 
seldom  requires  more  than  20 
minutes.  Leighty  and  Hutcheson 
(1919)  state  that  the  opening  of  the 
glumes  from  beginning  to  completion 
may  not  require  more  than  one 
minute,  that  the  anthers  may  be  ex- 
truded and  emptied  of  their  contents 
within  two  to  three  minutes  and  the 
glumes  again  become  tightly  closed 
at  the  end  of  15  to  20  minutes. 
Kirchner  (1886)  states  that  about 
one-third  of  the  pollen  falls  inside 
the  flower.  As  the  pollen  is  blown 
around  the  field  by  the  wind  it  is 
easily  seen  that  natural  crossing 
may  sometimes  occur. 

Investigators  differ  in  their  be- 
liefs regarding  natural  crossing  in 
small  grains.  De  Vries  (1906) 
says  "  wheat,  barley  and  oats  are 
self-fertile  and  do  not  mix  in  the 
field  through  cross-pollination . ' ' 
Biffin  (1905)  states  that  he  has  never 
observed  a  case  of  cross-pollination 
in  wheat;  while  Fruwirth  (1909)  lists 
several  German  breeders  who  have 
given  instances  of  natural  crosses.  Fruwirth  says 
ties  can  be  cultivated 
Nilsson-Ehle  (1915),  in  Sweden,  has  found  that  some  varieties 
show  a  much  greater  amount  of  natural  crossing  than  others. 
Howard  and  others  (19 10a),  in  India,  carefully  studied  natural 
crossing  in  wheat  for  several  years  and  recorded  231  natural 


FIG.  11. — Natural  wheat-rye 
hybrids.  Two  spikes  of  parent 
wheat  varieties  are  shown  on  the 
outside  with  hybrid  spikes  on  the 
inside.  (After  Leighty.) 


"  wheat  varie- 
side  by  side  for  years  without  mixing." 


36  BREEDING  CROP  PLANTS 

crosses.  Smith  (1912)  reported  eight  natural  hybrids  in  96  rows 
of  Turkey  winter  wheat  and  Saunders  (1905)  told  of  a  natural 
hybrid  which  occurred  at  Ottawa.  During  the  last  three  years 
at  University  Farm,  St.  Paul,  at  least  2  to  3  per  cent,  of  natural 
crossing  in  wheat  has  occurred  in  the  plant-breeding  plots. 
Cutler  (1919)  mentions  frequent  natural  crosses  at  Saskatoon, 
Canada. 

Barley. — Barley  frequently  is  self-fertilized  while  the  spike  is 
in  the  sheath.  In  four-rowed  barley  the  lateral  rows  overlap  in 
such  a  way  as  to  form  a  single  row  instead  of  two  rows  at  each 
edge  of  the  rachis,  as  in  the  normal  six-rowed  varieties.  Fru- 
wirth  (1909)  observed  natural  crosses  in  four-rowed  barleys  and 
concluded  there  was  practically  no  crossing  in  six-rowed  forms. 
He  records  the  observations  of  Rimpau,  who  noted  only  eight 
suspected  natural  crosses  in  barley  after  growing  40  varieties  side 
by  side  for  a  period  of  eight  years.  Harlan,  after  several  years' 
observation  at  University  Farm,  Minn,  noted  only  two  or  three 
natural  crosses.  Barley  probably,  therefore,  crosses  much  less 
frequently  than  does  wheat. 

Oats. — The  form  of  the  individual  flower  of  oats  is  very  similar 
to  that  of  wheat  and  barley.  Tschermak  (1901)  reports  four 
natural  crosses  observed  by  Rimpau,  and  Fruwirth  (1909)  records 
five  or  six  crosses  observed  by  Rimpau  after  cultivating  19 
varieties  side  by  side  for  eight  years.  A  natural  cross  between 
a  variety  of  Avena  sterilis  and  A .  nuda  was  noted  by  Pridham  in 
1916.  These  facts  and  numerous  statements  by  breeders  as  to 
self-fertilization  show  that  natural  crossing  occurs  much  less 
frequently  in  oats  than  in  wheat. 

Tobacco. — In  the  tobacco  plant  the  flowers  are  frequently 
visited  by  insects  and  some  natural  crossing  doubtless  takes 
place.  As  a  rule  only  one  variety  of  tobacco  is  grown  in  a  locality. 
Howard  and  others  (1910  b,  c),  in  India,  concluded  that  there  is 
between  2  and  3  per  cent,  crossing  in  tobacco.  They  emphasize 
the  necessity  of  producing  artificially  self-fertilized  seed.  In 
breeding  experiments,  artificially  selfed  seed  is  generally  used 
and  therefore  few  records  regarding  the  degree  of  cross-pollination 
are  available.  As  it  is  so  easy  artificially  to  self-fertilize  tobacco 
and  as  each  flower  produces  many  seeds  (98,910  seeds  per  plant, 
Jenkins,  1914)  the  amount  of  natural  cross-pollination  is  of  little 
breeding  importance. 

Flax. — The  flax  flower,  like  the  tobacco  flower,  is  frequently 


MODE  OF  REPRODUCTION  IN  RELATION  TO  BREEDING  37 

visited  by  insects  which  may  cause  natural  crossing.  Fruwirth 
(1909)  states  that  crossing  seldom  takes  place.  Howard  and 
others  (1910a)  have  observed  natural  crossing  under  Indian 
conditions.  Some  idea  of  the  frequency  of  natural  crosses  may 
be  gained  by  a  determination  of  the  percentage  of  selected  plants 
which  breed  true.  Results  of  this  nature  have  been  presented 
by  Howard  and  others  (1919). 

NUMBER  NUMBER 

YEAR  PLANTS  BREEDING 

SELECTED  TRUE 

1916  340  334 

1917  233  232 

1918  232  232 

Only  0.9  per  cent,  of  the  progeny  rows  showed  segregation. 

Rice. — In  rice  the  inflorescence  is  a  terminal  panicle  of  perfect 
flowers.  The  one-flowered  spikelet  has  a  branched  stigma  and 
six  stamens.  The  lodicules  are  strongly  developed.  Fruwirth 
(1909)  observed  the  period  of  blooming  in  rice  and  found  that 
30  seconds  elapsed  from  the  time  one  flower  began  to  open  until 
it  was  fully  open.  Dehiscence  of  the  anthers  occurred  about 
seven  minutes  later  and  the  flower  closed  three  hours  after.wards. 

In  rice  self-pollination  is  the  usual  method,  although  oppor- 
tunities for  crossing  occur.  Hector  (1913)  thinks  crosses  may 
occur  at  a  distance  of  not  more  than  2  ft.  by  the  agency  of  the 
wind.  In  lower  Bengal  4  per  cent,  of  crossing  was  estimated. 
Ikeno  (1914)  sowed  alternate  rows  of  blue-  and  white-seeded  rice. 
Xenia  occurs,  blue  being  dominant,  if  the  white-seeded  variety 
is  pollinated  by  the  blue.  Fifteen  thousand  kernels  from  190 
panicles  were  examined  and  no  xenia  was  found.  Thompstone 
(1915),  in  upper  Burma,  finds  that  pollination  usually  occurs  before 
the  glumes  open;  however,  hybrids  were  frequently  observed  in 
fields  of  ordinary  rice.  Parnell  and  others  (1918)  observed  the 
amount  of  natural  crossing  in  pure  green  plants  surrounded  by 
others  which  possessed  seed  with  a  purple  tip.  A  total  progeny 
of  nearly  15,000  plants  grown  from  seed  produced  by  the  green 
plants  were  observed,  more  than  2,000  plants  being  studied  in 
each  of  five  different  families.  The  percentage  of  crossing 
varied  from  0.1  per  cent,  in  one  variety  to  2.9  per  cent,  in  another. 
Alkemine  (1914)  states  that  cross-pollination  occurs  if  the 
anthers,  on  account  of  unfavorable  environmental  conditions,  do 
not  assume  their  natural  position.  This  happens  when  the  stig- 


38  BREEDING  CROP  PLANTS 

mas  protrude  from  the  glumes  and  take  a  pendent  position  before 
anther  dehiscence  takes  place. 

Cotton. — Probably  cotton  crosses  to  a  greater  extent  than  any 
of  the  other  plants,  except  sorghums,  listed  as  belonging  to  the 
naturally  self-fertilized  group.  Because  of  the  difference  in  ob- 
servations by  investigators  it  would  seem  that  varietal  differences 
are  one  probable  cause  for  the  discrepancies. 

Leake  (1911)  observed  5  per  cent,  natural  crossing  in  India. 
Figures  given  by  Webber  (1905)  and  Balls  (1912)  range  from  5 
to  13  per  cent. 

Grain  Sorghums. — Ball  (1910)  states: 

"All  sorghums  are  adapted  to  open  or  wind  pollination  and  most  of 
them  are  probably  adapted  to  self-fertilization.  In  adjacent  rows  of 
different  varieties  flowering  on  approximately  the  same  date,  as  high  as 
50  per  cent,  of  the  seed  produced  by  the  leeward  row  was  found  to  be 
cross-pollinated.  It  is  probable  that  in  a  fairly  uniform  field  of  any 
given  variety  a  similar  percentage  of  natural  crossing  takes  place." 

Graham  (1916),  in  India,  made  a  careful  study  of  the  amount 
of  cross-fertilization  in  the  Juar  plant  (Andropogon  sorghumBrot.). 
Crossing  was  more  frequent  in  the  looser  types  of  inflorescence 
than  in  the  compact  types.  Single  plant  cultures  were  used  for 
the  study,  which  extended  over  a  period  of  seven  years.  The 
percentage  of  crossing  obtained  by  counting  a  given  number  of 
plants  and  noting  those  which  were  untrue  to  type  gave  97  plants 
out  of  1,577  (6  per  cent.)  in  the  loose  headed  type  and  only  two 
plants  out  of  292  (0.6  per  cent.)  in  the  compact  type  of  panicle. 
Preliminary  studies  were  made  by  Karper  and  Conner  (1919) 
of  the  amount  of  cross-pollination  in  plants  of  white  milo  which 
were  found  growing  in  a  plot  of  yellow  milo.  The  yellow  and 
white  varieties  flowered  simultaneously.  Forty-one  heads  of 
white  milo,  which  had  been  surrounded  by  yellow  milo,  were 
planted  the  following  year.  An  average  of  6  per  cent,  of  natural 
crossing  in  plants  so  surrounded  was  noted. 

Peas  and  Beans. — Piper  (1912)  finds  that  natural  crossing  in 
the  cowpea  occurs  but  rarely  in  most  localities.  At  Arlington 
Farm,  in  the  experimental  plots,  instances  of  natural  crossing 
have  been  observed.  In  some  instances  natural  crossing  occurs 
more  frequently.  Thus  an  Indiana  farmer,  who  originally  grew 
only  eight  varieties,  found  after  several  years  that  he  had  over 
40  types.  The  new  types,  Piper  concluded,  were  the  result  of 


MODE  OF  REPRODUCTION  IN  RELATION  TO  BREEDING     39 

natural  crosses.  Similar  crosses  have  been  observed  at  the 
Michigan  station.  Harland  (1919)  has  recorded  a  supposed  case 
of  a  natural  cross  which  occurred  in  one  of  his  hybrid  cowpea 
families. 

Natural  hybrids  of  soybeans  have  been  observed  at  the  United 
States  experimental  farm  in  Virginia  and  also  at  the  Kansas 
experiment  station  (Piper  1916).  They  were  detected  by  the 
peculiar  color  of  their  seed.  Varieties  of  soybeans  were  inter- 
planted  at  the  Wisconsin  station  and  the  amount  of  natural 
crossing  was  determined  by  testing  the  progeny.  More  than 
10,000  plants  were  tested  and  only  a  fraction  of  1  per  cent,  of 
natural  crossing  was  found  (Russell  and  Morrison,  1919). 

Although  horticultural  peas  and  beans  are  largely  self-polli- 
nated, cross-pollination  does  occasionally  occur.  Howard  and 
others  (1910a)  give  observations  in  India  which  indicate  natural 
crosses  both  in  garden  and  field  peas. 

Tomatoes. — Jones  (1916)  planted  alternate  plants  of  dwarf  and 
standard  varieties  of  tomatoes  3  ft.  apart  in  a  field.  Seed  from 
the  dwarfs  was  tested  the  following  year.  As  standard  habit  is 
a  dominant  character,  pollen  from  a  standard  plant  fertilizing  a 
dwarf  would  give  a  standard  in  F\. 

A  total  of  2,170  plants  were  grown  from  seed  of  dwarfs  and 
43  proved  to  be  standards.  This  is  practically  2  per  cent.  As 
there  was  nearly  as  great  opportunity  for  dwarfs  to  be  crossed 
with  dwarf  pollen  it  would  seem  that  between  3  and  4  per  cent,  of 
crossing  occurred  in  this  experiment. 

THE  OFTEN  CROSS-POLLINATED  PLANTS 

Maize. — Maize  has  been  placed  at  the  head  of  the  often 
cross-pollinated  group,  as  crossing  is  its  normal  form  of  repro- 
duction. Fruwirth  (1909)  found  a  setting  of  24  per  cent,  in  un- 
enclosed corn  plants  when  far  enough  from  other  plants  to  prevent 
crossing.  Knuth  (1909),  in  similar  experiments,  found  16  per 
cent,  selfing  on  the  upper  ear  and  4  per  cent,  on  the  lower. 
Preliminary  experiments  have  been  made  by  planting  corn  with  a 
recessive  endosperm  color  in  a  field  of  a  variety  with  a  dominant 
endosperm  character.  Self-fertilization  in  these  experiments 
was  probably  less  than  5  per  cent.  (Waller,  1917,  Hayes,  19186). 

Rye. — The  flowers  of  rye  are  very  similar  to  those  of  wheat 
and  barley.  According  to  Hildebrand  the  anthers  project 


40 


BREEDING  CROP  PLANTS 


between  the  partly  closed  glumes  until  the  bases  protrude. 
They  then  tip  over  and  dehisce,  spilling  part  of  the  pollen  outside 
the  flower.  Being  lower  than  the  stigma  the  pollen  can  not  reach 
the  stigma  of  the  same  flower.  There  is  some  evidence  (Ulrich, 
1902)  (Fruwirth,  1909)  which  indicates  that  the  rye  flower  is 
self-sterile,  but  that  the  spikelet  is  not  necessarily  so.  Further 
studies  are  needed  to  clear  up  this  point. 

Ulrich  (1902)  found  significant  differences  between  varieties 
and  individuals  of  the  same  variety  in  the  amount  of  self -sterility. 
The  following  table  shows  some  of  his  results,  obtained  from 
artificial  and  natural  pollination.  Artificial  pollination  was 
obtained  by  covering  the  head  with  double  paper  bags. 

TABLE  II. — SELF-STERILITY  IN  RYE 


Variety 

Artificial  pollination 

Natural 
pollination, 
per  cent. 

Individual 
spikes, 
per  cent. 

Groups  of  more  than  1  spike 

Same  plant, 
per  cent. 

Dif.  plant, 
per  cent. 

Petkuser 

1.30 
2.33 
5.02 

2.52 

4.98 
7.21 

28.29 

25.12 
38.32 

80 
59 

78 

Probsteier  

Schlanstedter  

Heribert  Nilsson  (1916)  isolated  lines  in  Petkuser  rye  differing 
greatly  in  amount  of  self -sterility.  Of  73  plant  selections,  71 
were  practically  self -sterile,  one  showed  segregation,  and  one 
proved  to  be  highly  self -fertile.  The  rye  flower  is  probably 
largely  cross-pollinated  and  because  of  the  heterozygous  condi- 
tion, strains  differing  in  fertility  make  up  any  particular  variety. 

Alfalfa. — Piper  and  others  (1914)  working  with  alfalfa  have 
found  about  the  same  percentages  of  seed  set  when  a  flower 
was  self-pollinated  as  when  it  was  crossed  with  pollen  from 
flowers  on  the  same  plant.  When  cross-pollination  was  prac- 
ticed, approximately  50  per  cent,  more  seed  was  obtained  than 
from  self-fertilization.  They  also  found  that  pollen  of  Medicago 
falcata  was  as  efficient  in  fertilizing  M.  saliva  as  pollen  from  other 
saliva  plants. 

Waldron  (1919),  in  North  Dakota,  planted  together  in  equal 
numbers  two  species  of  Medicago,  saliva  and  falcata.  Seeds 
from  each  of  the  species  were  planted  the  following  year  and  the 
number  of  hybrids  noted.  From  M.  falcata  42.7  per  cent,  of 


MODE  OF  REPRODUCTION  IN  RELATION  TO  BREEDING     41 


hybrid  plants  were  obtained  and  from  the  M.  sativa  seed  about 
7.5  per  cent.  A  part  of  the  difference  in  the  results  is  doubtless 
due  to  the  fact  that  the  plants  produce  a  smaller  number  of 
flowers  and  are  procumbent  to  prostrate  in  habit.  To  find  the 
amount  of  cross-pollination  that  normally  occurs  in  alfalfa,  one 
might  average  the  above  results  and  multiply  the  result  by  two. 
This  gives  in  the  neighborhood  of  50  per  cent,  of  natural  crossing 
which  is  only  indicative  of  the  probable  amount. 

Grasses. — Some  studies  with  grasses  have  been  reported  by 
Frandsen  (1917).  Results  obtained  are  given  in  the  following 
table.  Some  sterility  is  indicated  by  comparing  the  results  of 
self-fertilization  with  those  of  cross-fertilization  and  natural 
pollination.  Poa  fertilis  and  Bromus  arvensis  appear  self-fertile. 
Considerable  self-sterility  is  indicated  in  orchard  grass,  timothy, 
and  fescue. 

TABLE  III. — POLLINATION  OF  GRASSES 


Common  name 

Scientific  name 

Percentage  seed  setting 

Self: 
fertilizing 

Cross- 
fertilizing 

Free- 
flowering 

Orchard  
Tall  meadow  oat 
Fescue 

Dactylis  glomerata  
Arrhenatherum  elatius  . 
Festuca  pratensis  
Alopecurus  pratensis..  . 
Lolium  multiflorum.  .  .  . 
Phleum  pratensis 

1.3-11.5 
5.4-9.4 
3.6-9.2 
7.0-23.3 
10.3 
0.8-  8.5 
59.7-66.8 
66.6-80.0 

4.3-75.8 
47.9 
17.8-54.0 
29.0-69.5 

50.0 
51.0 
35.2-47.7 
73.2 
79.8 
91.3 
70.4 
77.2-89.2 

Meadow  foxtail.  . 
Italian  rye  
Timothy 

52.0 
63.5-65.5 
80.4 

Brome  

Poa  fertilis  
Bromus  arvensis  

EFFECTS  OF  A  CROSS  IN  NORMALLY  SELF -FERTILIZED  SPECIES 

A  cross  between  closely  related  varieties  frequently  exhibits  a 
quite  marked  increase  in  vigor  when  compared  with  the  parents. 
This  is  a  manifestation  of  the  same  phenomenon  as  decrease  in 
vigor  which  is  commonly  the  result  of  self -fertilizing  a  naturally 
cross-fertilized  species.  With  self -fertilized  crops  it  is  usually 
not  possible  to  utilize  this  increased  vigor  because  the  cost  of 
producing  crossed  seed  is  too  great.  Examples  of  FI  crosses  in 
tomato,  tobacco,  and  wheat  will  be  given. 

Table   IV  gives  the  comparative  yields  of  first  generation 


42 


BREEDING  CROP  PLANTS 


tobacco  crosses  and  their  parents.  All  crosses  do  not  prove 
equally  vigorous  and  a  few  give  no  increase  as  compared  with  the 
parental  average.  In  general,  however,  the  crosses  show  in- 
creased yields.  As  the  tobacco  flower  produces  many  seeds, 
Houser  (1912)  believes  the  extra  cost  of  production  would  not 
be  prohibitive.  Before  this  plan  can  be  adopted  commercially, 
extensive  studies  are  needed  to  determine  the  value  of  particular 
Fi  tobacco  crosses. 


TABLE  IV. — RELATION  OF  YIELD  PER  ACRE  BETWEEN  FIRST  GENERATION 
HYBRID  TOBACCO  AND  THE  PARENT  PLANTS 


Average  yield  of 

Average  increase  of  hybrid 

Maximum  increase  of  hybrid 

parents,  Ib. 

over  parents,  Ib. 

over  parents,  Ib. 

800-    900 

260                                       485 

901-1,000 

212                                       464 

1,001-1,100 

185 

354 

1,101-1,200 

153 

315 

1,201-1,300 

153 

285 

1,301-1,400 

159 

239 

over  1,400 

156 

189 

Difference  in 

yield  of  parents 

1-100 

197 

485 

101-200 

131 

181 

201-300 

189 

260 

301-400 

97 

360 

401-500 

164 

215 

over  500 

175 

465 

The  vigor  of  FI  tomato  crosses  has  received  some  study.  The 
first  extensive  test  was  made  by  Wellington  (1912)  at  the  Geneva 
(New  York)  Station.  A  3-year  test  was  made  under  field  con- 
ditions of  a  cross  between  Dwarf  Aristocrat,  a  dwarf  tomato, 
and  Livingston  Stone.  Yields  of  the  parents,  the  FI,  and  the  F% 
generations  are  given. 

We  are  not  so  much  interested  at  the  present  time  in  the  com- 
mercial value  of  such  crosses  as  in  the  development  of  the  princi- 
ple involved.  Wellington  believes  the  above  cross  of  sufficient 
value  to  more  than  pay  for  the  cost  of  producing  crossed  seed. 

Similar  results  were  obtained  at  the  Connecticut  Station  in  a 
cross  between  Stone  and  Dwarf  Champion  tomatoes.  The 


MODE  OF  REPRODUCTION  IN  RELATION  TO  BREEDING  43 


TABLE  V. — YIELDS  OF  FRUIT  IN  THE  FI  AND  F2  GENERATIONS  OF  A  CROSS 
BETWEEN  DWARF  ARISTOCRAT  AND  LIVINGSTON  STONE  WITH  THE 

PARENTS 


Data  taken 

Year 

Dwart 
Aristocrat, 
Ib. 

Livingston 
Stone,  Ib. 

*•», 

Ib. 

Fy, 
Ib. 

Ripe  fruit  per  plant  
Ripe  fruit  per  plant  
Ripe  fruit  per  plant  

1908 

1909 
1910 

8.5 

6.1 
7.0 

12.3 
10.1 
12.0 

13.9 
12.9 
13.2 

12.0 
10.0 

Average 

7.2 

11.5 

13.3 

11.0 

Total  fruit  per  plant  
Total  fruit  per  plant  
Total  fruit  per  plant  
Average 

1908 
1909 
,1910 

14.8 
9.7 
14.8 
13.1 

20.9 
17.7 
24.7 
21.1 

25  3 
20.0 

27.7 
24.3 

20.0 
25.1 
22.6 

experiment  was  carried  on  for  four  years  (Hayes  and  Jones,  1916). 
The  lowest  increase  in  yield  over  the  better  parent  was  11  per 
cent,  and  the  highest  17  per  cent.  The  cross  averaged  15  per 
cent,  more  fruit  by  weight  than  the  better  parent. 

In  average  weight  of  fruit  the  cross  exceeded  the  parental 
average  by  8  per  cent.  It  approached  the  fruit  number  of  the 
Dwarf  Champion  parent  and  exceeded  the  average  fruit  number 
of  the  parents  by  8  per  cent.  The  cross  also  matured  somewhat 
earlier  than  the  early  parent.  A  cross  between  the  standard 
varieties,  Lorillard  and  Best  of  All,  was  also  studied.  The 
parents  produced  about  the  same  average  size  and  weight  of 
fruit  and  the  cross  about  the  same  as  the  parents. 

A  determination  of  the  comparative  vigor  of  F\  wheat  crosses 
and  their  parents  was  made  by  Fred  Griffee,  a  graduate  student  in 
plant  breeding  at  the  University  of  Minnesota.  For  this  purpose 
pure  lines  of  T.  durum,  T.  dicoccum  and  T.  compactum  were 
crossed  with  pure  line  varieties  of  T.  vulgare.  Intervarietal 
crosses  between  pure  lines  of  T.  vulgare  were  also  studied,  as  well 
as  crosses  between  T.  compactum  with  T.  durum  and  T.  dicoccum. 

A  determination  of  the  immediate  effect  of  foreign  pollen  on 
size  of  seed  was  made.  Parental  plants  were  emasculated  and 
then  some  of  the  spikes  were  artificially  pollinated  with  pollen 
from  other  plants  of  the  same  pure  line  (incrossed  seed)  and 
in  another  series  spikes  were  pollinated  with  pollen  from  another 
variety  or  species  (crossed  seed).  Only  those  crosses  were  com- 
pared in  which  the  average  date  of  pollination  was  about  the 


44 


BREEDING  CROP  PLANTS 


same  for  the  incrossed  and  crossed  seed.     Results  are  presented 
in  Table  VI.  .^, 

TABLE  VI. — WEIGHT   OF  SEED  OF  INCROSSED  PARENTS  COMPARED  WITH 
WEIGHT  OF  THE  IMMEDIATE  CROSS 


9 

Parent 

Cross 

Name  of  cross 

No. 

seeds 

Average 
weight 
seed,  mg. 

No. 

seeds 

Average 
weight 
seed,  mg. 

cross-female 
parent 

Marquis  X  Velvet  Chaff  
Marquis  X  Penny 

38 
38 

12.6±0.5 
12  6  +  0  5 

48 
24 

15.6±0.5 
20  2  +  1  0 

+3.0±0.7 
4-7  6  +  1    1 

Haynes  Bluestem  X  Marquis  
Little  Club  X  Marquis  

49 
39 

17.2  +  0.8 
10  .  1  ±  0  5 

26 
50 

23.5±0.7 
94  +  03 

+  6.3±1.1 
—  0  7  +  0  6 

Emmer  X  Velvet  Chaff  
Velvet  Chaff  X  Mindum 

44 
104 

26.4±0.8 
19  9  +  0  6 

24 
23 

27.1±1.3 
15  9±0  6 

+0.7±1.5 
—  4  0  +  08 

Emmer  X  Little  Club  

44 

26.4±0.8 

15 

25.0±1.2 

-1.4±1.4 

All  three  crosses  between  varieties  of  T.  vulgare  gave  increases 
over  incrossed  seed.  These  appear  significant  in  relation  to  the 
computed  probable  errors.  Of  the  crosses  between  wheat  species 
only  one  gave  a  difference  which  appears  at  all  significant.  In 
the  cross  between  Velvet  Chaff  and  Mindum  the  incrossed  seed 
seems  somewhat  heavier  in  the  light  of  the  probable  error  than 
the  crossed  seed.  These  results  show  an  immediate  effect  of 
pollination  on  seed  size  in  crosses  between  varieties  of  T.  vulgare. 

The  emasculation  and  artificial  pollination  causes  a  reduction 
in  seed  size  as  compared  with  normally  produced  seed.  In- 
crossed,  normally  produced  seed  and  crossed  seed  were  grown  in 
the  greenhouse  under  controlled  conditions  and  the  comparative 
vigor  of  parents  and  crosses  was  determined.  As  there  were  no 
significant  correlations  between  size  of  seed  planted  (even  when 
incrossed  seed  was  compared  with  normal  seed)  and  resultant 
plant  vigor,  the  differences  between  the  parents  and  crosses  may 
be  explained  on  the  basis  of  inheritance. 

Average  yield  of  plants  in  grams  of  seed  will  be  used  as  a 
measure  of  vigor  (see  Table  VII). 

The  crosses  between  varieties  of  T.  vulgare  and  the  crosses 
between  T.  vulgare  and  T.  compactum  gave  in  every  case  slightly 
greater  yields  per  plant  than  the  average  of  the  parents.  On 
the  other  hand,  F\  crosses  between  durum  or  emmer  varieties 
and  varieties  of  common  or  club  wheats  were  all  significantly 
lower  in  yield  than  the  parents.  The  low  yields  of  these  species 


MODE  OF  REPRODUCTION  IN  RELATION  TO  BREEDING     45 


TABLE   VII. — AVERAGE    YIELD   PER   PLANT  OF  Fi   WHEAT   CROSSES  AND 

THEIR  PARENTS 


Aver- 

Crc 

ss 

Name  of  one 
parent 

No.  of 
indi- 
viduals 

Yield, 
grains 

Name  of 
other  parent 

indi- 
viduals 

Yield, 
grams 

age 
weight 
parents, 

No.  of 
indi- 

Yield, 

grams 

viduals 

Marquis  

15 

1.9 

Penny  

36 

2.4 

2.2 

18 

2.7 

Marquis  

15 

.9 

Bobs  

59 

3.0 

2.5 

65 

3.3 

Velvet  Chaff  

38 

.5 

Penny  

36 

2.4 

2.0 

28 

2.5 

Velvet  Chaff  

38 

.5 

Bobs  

59 

3.0 

2.3 

92 

2.9 

Penny  

36 

.4 

Bobs  

59 

3.0 

2.7 

23 

2.8 

Haynes  Bluestem  .  . 

47 

.4 

Marquis  

15 

1.9 

2.2 

18 

2.5 

Marquis  

15 

.9 

Little  Club  .  . 

46 

2.2 

2.1 

45 

2.3 

Velvet  Chaff  

38 

.5 

Little  Club  .  . 

46 

2.2 

1.9 

37 

2.5 

Average  

30 

1.9 

45 

2.5 

2.2 

41 

2.7 

Little  Club  

46 

2.2 

Emmer  

48 

1.1 

1.7 

9 

0.3 

Little  Club  

46 

2.2 

Mindum  .... 

49 

2.1 

2.2 

1 

1.0 

Marquis  

15 

1.9 

Mindum  .... 

49 

2.1 

2.0 

13 

0.3 

Velvet  Chaff  

38 

1.5 

Mindum  .... 

49 

2.1 

1.8 

8 

1.1 

Velvet  Chaff  

38 

1.5 

Emmer  

48 

1.1 

1.3 

23 

0.5 

Marquis  

15 

1.9 

Emmer  

48 

1.1 

1.5 

18 

0.6 

Average  

33 

1.9 

49 

1.6 

1.8 

12 

0.6 

crosses  are  due  in  a  large  measure  to  sterility  for  there  was  an 
appreciably  smaller  setting  of  seeds  in  the  crosses  than  in  their 
parents. 

Crosses  between  distinct  species  of  self-fertilized  plants  have 
been  carefully  studied  in  the  tobacco  genus,  Nicotiana.  Results 
obtained  may  be  summed  up  as  follows  (East  and  Hayes,  1912): 

"(a)  plants  so  different  that  they  will  not  cross;  (6)  crosses  that  pro- 
duce seed  that  contain  no  proper  embryo ;  (c)  crosses  that  produce  seed 
with  embryo,  but  which  go  no  further  than  the  resting  stage  of  the  seed; 
(d)  crosses  less  vigorous  than  either  parent;  (e)  crosses  more  vigorous 
than  the  average  of  the  parents;  and  (/)  crosses  more  vigorous  than 
either  parent." 

Apparently  in  wide  crosses  the  normal  physiological  processes 
are  interfered  with.  The  statement  is  frequently  made  that 
this  is  due  to  lack  of  compatibility  between  the  parents.  The 
specific  physiological  cause  is  not  yet  known. 

EFFECTS  OF  SELF-FERTILIZATION  IN  NORMALLY  CROSS- 
FERTILIZED  PLANTS 

This  subject  will  be  studied  in  relation  to  the  specific 
outline  for  breeding  some  normally  cross-fertilized  plants, 
such  as  maize  and  rye.  A  few  data  will  be  presented  in 


46 


BREEDING  CROP  PLANTS 


order  to  illustrate  the  general  results.  The  theoretical  explana- 
tion is  given,  as  an  appreciation  of  these  phenomena  is  essential 
in  obtaining  a  correct  plant  breeding  perspective. 

The  most  extensive  studies  made  have  been  those  with  maize. 
As  this  crop  is  almost  entirely  cross-pollinated  under  natural 
field  conditions  it  is  an  admirable  one  to  contrast  with  self- 
fertilized  plants.  Table  VIII  presents  differences  in  yield  and 
height  obtained  at  the  Connecticut  Station  with  four  self-fer- 
tilized strains  of  Learning  Dent.  These  strains  were  grown  only 
in  small  plots,  therefore  differences  are  only  indicative  of  the 
general  results  which  may  be  expected.  Crosses  between  in- 
dividual plants  within  a  strain  that  had  been  selfed  six  or  seven 
years,  were  not  appreciably  more  vigorous  than  the  progeny  of 
self -fertilized  seed.  These  strains  also  differ  in  other  characters, 
such  as  shape  of  ear,  width  of  leaf,  and  color  in  various  organs. 
One  strain  of  Learning  Dent  No.  1-12  was  self-fertilized  for  about 
seven  years.  It  produced  well-developed  tassels  but  few  ears 
and  was  eventually  lost. 

TABLE  VIII. — THE  EFFECT  OF  INBREEDING  ON  THE  YIELD  AND  HEIGHT  OF 

MAIZE 


Year 
grown 

No.  of 
genera- 
tions 
selfed 

0 

The  four  strains 

1-6-1-3,  etc. 

1-7-1-1,  etc. 

1-7-1-2,  etc. 

1-9-1-2,  etc. 

Yield, 
bu.  per 
acre 

Height, 
in. 

Yield, 
bu.  per 
acre 

Height, 
in. 

Yield, 
bu.  per 
acre 

Height, 
in. 

Yield, 
bu.  per 
acre 

Height, 
in. 

1916 

74.7 

117.3 

74.7     ;    117.3 

74.7         117.3 

74.7 

117.3 

1905 

0 

88.0 

88.0 

88.0 

88.0 

1906 

1 

59.1 

60.9 

60.9 

42.3 

1907 

1907 

1908 

2 

95.2 

59.3 

59.3 

51.7 

1908 

1908 

1909 

3 

57.9 

46.0 

59.7 

35.4 

1910 

4 

80.0 

63.2 

68.1 

47.7 

1911 

5 

27.7 

86.7 

25.4 

81.1 

41.3 

90.5 

26.0 

76.5 

1912 

6 

1913 
38.9 
1914 

1913 

7 

41.8 

39.4 

45.4 

85.0 

1915 

1914 

8 

78.8 

96.0 

47.2 

83.5 

58.5 

88.0 

21.6 

1916 

1915 

9 

25.5 

24.8 

30.6 

78.7 

1917 

1916 

10 

32.8 

97.7 

32.7 

84.9 

19.2 

86.9 

31.8 

82.4 

1917 

11 

46.2 

103.7 

42.3 

78.6 

37.6 

83.8 

i 

MODE  OF  REPRODUCTION  IN  RELATION  TO  BREEDING     47 

From  these  and  other  results  (Jones,  1918)  it  is  apparent  that 
selfing  in  maize  produces: 

1.  Strains  which  can  not  be  perpetuated. 

2.  Strains  which  can  be  perpetuated  only  with  difficulty. 

3.  Strains  which  exhibit  normal  development  but  vary  in  amount  of 
growth  attained. 

EXPLANATION  OF  HYBRID  VIGOR1 

The  studies  of  the  early  hybridizers,  Koelreuter,  Gartner, 
Knight,  and  others,  gave  results  which  can  be  summed  up  in  a 
single  sentence  as  follows  (East  and  Hayes,  1912) : 

"  Crosses  between  varieties  or  between  species  often  give  hybrids  with 
a  greater  vegetative  vigor  than  is  possessed  by  either  parent." 

Darwin  made  extended  and  careful  studies  of  the  effects  of 
cross-  and  self-fertilization  in  plants.  He  conclusively  proved 
that  in  general  there  is  an  advantage  in  cross-fertilization.  While 
he  noted  some  self-fertilized  families  he  believed  these  would 
eventually  perish.  Lacking  as  he  did  a  knowledge  of  Mendelian 
phenomena  it  was  impossible  for  Darwin  to  develop  as  logical 
an  explanation  of  these  results  as  we  now  have.  Darwin  thought 
the  results  could  best  be  explained  by  the  nature  of  the  sexual 
elements  rather  than  in  the  act  of  crossing. 

Several  explanations  of  hybrid  vigor  have  been  advanced 
since  the  rediscovery  of  Mendel's  law.  In  all  cases  hete'rozygosis 
has  received  a  major  place  in  the  explanation.  The  results  of 
these  studies  have  been  summed  up  as  follows  (East  and  Hayes, 
1912): 

"1.  The  decrease  in  vigor  due  to  inbreeding  naturally  cross-fertilized 
species  and  the  increase  in  vigor  due  to  crossing  naturally  self-fertilized 
species  are  manifestations  of  one  phenomenon.  This  phenomenon  is 
heterozygosis.  Crossing  produces  heterozygosis  in  all  characters  by 
which  the  parent  plants  differ.  Inbreeding  tends  to  produce  homozy- 
gosis  automatically. 

"  2.  The  phenomenon  exists  and  is  in  fact  widespread  in  the  vegetable 
kingdom. 

"3.  Inbreeding  is  not  injurious  in  itself,  but  weak  types  kept  in 

1  A  recent  monograph  by  EAST  and  JONES  (1919)  presents  in  a  clear  and 
concise  way  the  effects  of  inbreeding  and  cross-breeding  in  the  light  of 
modern  theories  of  genetics.  This  publication  has  been  used  very  freely 
in  this  section. 


48  BREEDING  CROP  PLANTS 

existence  in  a  cross-fertilized  species  through  heterozygosis  may  be 
isolated  by  its  means.  Weak  types  appear  in  self-fertilized  species,  but 
are  eliminated  because  they  must  stand  or  fall  by  their  own  merits." 

Biologists  commonly  believe  that  internal  or  external  agencies 
do  occasionally  modify  the  germ  plasm.  It  is  also  commonly 
accepted  that  somatic  modifications  do  not  impress  themselves 
upon  the  germ  plasm.  From  the  facts  of  segregation  as  explained 
by  the  Mendelian  law  and  the  acceptance  of  the  theory  of  factor 
stability,  we  may  next  consider  what  may  be  expected  in  self- 
pollinating  a  naturally  cross-fertilized  plant,  such  as  corn,  or 
what  will  result  in  later  generations  after  making  a  cross  in 
naturally  self-fertilized  plants. 

Several  slightly  different  formulae  have  been  advanced  to  show 
the  theoretical  expectation.  The  simplest  formula  for  the  per- 
centage of  homozygous  types  in  any  generation  following  a  cross 

/2n—  l\m 
between  different  forms  is  f  )   .     In  this  formula  n  is  the 

number  of  segregating  generations  which  has  elapsed  since  the  cross 
was  made  and  m  is  the  number  of  separately  inherited  allelomor- 
phic  pairs  of  factors  involved.  In  self-fertilized  organisms  this 
would  not  absolutely  hold  unless  all  the  progeny  of  each  genotype 
were  equally  productive  numerically. 

In  artificially  self-fertilizing  naturally  cross-pollinated  plants, 
such  as  corn,  it  is  theoretically  possible  to  select  a  completely 
heterozygous  individual  in  each  generation  for  self-fertilization 
and  thus  obtain  no  reduction  in  heterozygosis.  Jones  (1919) 
has  worked  out  theoretical  curves  for  1,  5,  10,  and  15  allelo- 
morphic  pairs  of  factors  for  from  one  to  10  generations  of  self- 
fertilization  following  a  cross. 

Some  facts  regarding  the  effects  of  self-fertilization  in  genera- 
tions following  a  cross  are  apparent  from  a  consideration  of  this 
figure.  When  only  a  single  allelomorphic  pair  is  concerned,  the 
first  generation  of  selfing  reduces  the  percentage  of  heterozygous 
individuals  by  half.  When  a  number  of  factor  pairs  are  con- 
cerned reduction  of  the  percentage  of  heterozygous  individuals  is 
comparatively  slow  for  the  first  few  years  of  selfing.  At  the  end 
of  10  years  the  percentage  of  heterozygotes  is  veiy  low  whether 
the  Initial  cross  was  heterozygous  for  15  allelomorphic  pairs  or 
for  a  single  allelomorphic  pair.  From  the  above  discussion  it  is 
apparent  that  after  several  years  of  self-fertilization  following  a 


MODE  OF  REPRODUCTION  IN  RELATION  TO  BREEDING     49 

cross  between  different  varieties  a  large  percentage  of  the  plants 
are  homozygous  and  will  breed  true  for  their  characters  if  self- 
fertilization  is  continued.  The  number  of  different  biotypes 
which  can  be  isolated  from  a  cross  depends  upon  the  number  of 
allelomorphic  pairs  of  factors  involved  and  their  linkage  relations. 
Formerly  the  heterozygous  condition  was  believed  to  carry 
with  it  an  increased  developmental  stimulus.  It  was  also  believed 
that  this  stimulus  was  greater  when  the  mate  to  an  allelomorphic 


100* 


Percentage  of  Heterozygous 
Individuals  in  each  Selfed 
Generation  when  the  Number 
of  Allelomorphs  Concerned 
Are:  1,5,10,15. 


456 

Segregating  Generations 


10 


FIG.  12. — The  percentage  of  heterozygous  individuals  and  the  percentage  of 
heterozygous  allelomorphic  pairs  in  the  whole  population  in  each  generation 
of  self-fertilization.  (After  Jones.) 

pair  was  lacking  than  when  both  were  present.  The  physiolog- 
ical cause  of  this  growth  stimulus  was  not  known  although  it 
was  recognized  that  "the  greater  the  degree  of  heterozygosis  the 
greater  is  the  vigor  of  the  resulting  plant"  (East  and  Hayes,  1912). 
A  considerable  number  of  studies  showed  that  the  rapidity  and 
amount  of  cell  division  was  increased. 

A  Mendelian  explanation  of  this  growth  stimulus  which  is  so 
frequently  found  in  crosses,  has  been  advanced.  Jones  (1918)  has 
explained  the  vigor  of  F\  which  has  been  called  heterosis  on  the 

4 


50  BREEDING  CROP  PLANTS 

basis  of  dominance  and  linkage.  In  comparing  crosses  with  their 
parents  it  is  quite  common  to  find  that  the  F\  generation  has  a 
higher  value  for  nearly  every  growth  character  than  has  the  aver- 
age of  the  parents.  Modern  geneticists  recognize  that  each 
character  is  due  to  the  interaction  of  many  inherited  factors.  If 
each  growth  factor  gives  as  great  an  effect  when  heterozygous  as 
when  homozygous  or  proves  partially  dominant  when  heterozy- 
gous, it  would  be  easy  to  explain  heterosis  by  the  actual  physio- 
logicial  growth  development  which  is  a  part  of  the  normal 
expression  of  a  particular  inherited  factor.  This  explanation 
was  formerly  advanced  to  account  for  heterosis  but  was  con- 
sidered unreliable,  as  it  was  difficult  to  account  for  the  almost 
universal  decrease  in  vigor  when  such  plants  as  maize  were 
selfed.  This  can  be  explained  by  the  facts  of  linkage,  as  it  is 
possible  to  have  a  greater  number  of  different  growth  factors 
present  in  a  heterozygous  than  in  a  homozygous  individual. 
The  explanation  has  much  in  its  favor. 


CHAPTER  IV 
FIELD  PLOT  TECHNIC 

In  carrying  out  crop-breeding  studies  the  number  of  varieties 
and  strains  has  been  greatly  multiplied.  Vilmorin's  isolation 
principle,  whereby  the  value  of  any  selection  is  determined  by  the 
breeding  nature  of  the  progeny,  has  been  universally  adopted. 
The  field  is  then  the  plant-breeder's  laboratory  and  the  question 
of  correct  field  technic  is  of  the  utmost  importance. 

The  difficulties,  of  making  all  conditions  of  similar  nature  for  a 
large  number  of  strains  or  varieties  which  must  be  tested,  are 
very  numerous.  The  method  used  must  be  such  that  the  per- 
formance will  be  a  correct  indication  of  the  comparative  value 
of  the  strains  being  tested  when  grown  under  farming  con- 
ditions. The  purpose  of  the  present  chapter  is  to  discuss  field 
plot  technic  for  such  disturbing  factors  as  soil  heterogeneity  and 
climatic  conditions. 

SOIL  HETEROGENEITY 

The  field  selected  for  the  comparative  trials  should  be  repre- 
sentative of  the  soil  and  climatic  conditions  under  which  the  crop 
will  be  grown.  The  land  must  then  be  cropped  in  such  a  manner 
that  it  is  kept  in  a  uniform  state  of  good  productivity.  In  order 
to  do  this,  it  is  necessary  to  observe  some  one  of  the  standard 
rotations.  It  is  a  good  practice  to  have  one  or  more  bulk  crops 
rotated  with  the  breeding  plots  in  order  to  keep  the  land  uniform. 
If  only  one  area  of  land  is  available  there  is  then  no  choice  and  the 
investigator  must  see  that  this  field  is  treated  in  the  best  possible 
way.  If  more  than  one  field  is  available,  it  is  possible  to  deter- 
mine which  is  more  nearly  uniform  by  a  correlation  of  contiguously 
grouped  plots  as  outlined  by  Harris  (1915). 

Harris'  Method  of  Estimating  Soil  Heterogeneity. — By 
Harris'  method  the  coefficient  of  correlation  is  used  as  an  index 
of  soil  uniformity  .  This  statistical  constant  measures  the  degree 
of  correlation  between  contiguous  plots  grouped  in  a  certain  way. 
If  the  variation  in  yield  from  plot  to  plot  is  simply  due  to  random 
sampling,  there  will  be  no  correspondence  between  contiguously 
grouped  units.  On  the  other  hand,  if  the  field  is  "patchy" 

51 


52 


BREEDING  CROP  PLANTS 


certain  contiguous  units  tend  to  yield  high  while  others  show  a 
tendency  in  the  opposite  direction.  Under  these  conditions  a 
high  correlation  coefficient  results.  If  variability  due  to  random 
sampling  only  is  entering,  the  correspondence  between  some 
contiguous  plots  will  be  counterbalanced  by  the  lack  of  corre- 
spondence between  others,  providing  that  the  number  of  ultimate 
units  is  sufficiently  large  to  permit  an  expression  of  the  law  of 
average.  It  is  obvious  that  in  the  application  of  Harris'  method 
the  field  must  receive  the  same  treatment  (seed,  cultivation, 
fertilizer,  etc.).  The  division  of  the  field  into  the  desired  units 
may  be  made  at  any  time  before  the  crop  is  harvested,  but 
preferably  before  or  soon  after  planting  in  order  to  minimize 
possible  injury  to  the  growing  crop. 

A  simple  illustration  will  make  the  calculation  of  the  correla- 
tion coefficient  clear,  although  a  much  larger  number  of  units 
should  be  used  in  an  actual  study  of  the  reliability  of  a  field  for 
plot  work.  Suppose  a  certain  field  is  divided  into  16  units  and 
these  units  are  in  turn  arranged  in  groups.  Let  p\,  p2,  Pz,  etc., 
represent  the  ultimate  units  and  CPl,  CPV  etc.,  represent  the 
groups.  By  assigning  values  for  yield  in  bushels  per  acre  to  the 
ultimate  units,  one  may  make  the  calculation  necessary  to  apply 
the  formula.  The  value  of  any  particular  group  is  the  sum  of 
the  ultimate  units  in  it. 

DIAGRAM  ILLUSTRATING  HARRIS'  METHOD 


(2) 

(2) 

(4) 

(6) 

Pi 
C 

P2 
Pi 

Ps 
C 

P4 
P2 

(3) 

(3) 

(6) 

(4) 

P5 

Pe 

P^ 

Ps 

(3) 

(3) 

(5) 

(5) 

P9 

C 

PIO 

P3 

Pii 
C 

Pl2 
P4 

(5) 

(5) 

(4) 

(4) 

Pis 

Pu 

Pii 

Pl6 

p  =  Average  yield  of  all  ultimate  units  = 
n  =  Number  of  units  in  each  group  = 
n  =  Number  of  groups  = 

=  Sum  of  squares  of  the  yields  assigned 
for  ultimate  units  = 


4 
4 
4 

280 

S(CP2)  =  Sum  of  squares  of  the  group  yields  =  1,080 
<TP  =  Standard  deviation  of  assigned  yield 

for  the  ultimate  units  =  vT5  =  1.2247 

<V>  =  (1.2247)2  =  1.4999 


The  numbers  enclosed  in  parentheses  represent  assumed  values  (bushels 
per  acre). 

Now  according  to  the  formula 


FIELD  PLOT  TECHNIC  53 

Where  rPlP2  is  the  constant  sought,  S  is  indicative  of  summation, 
CP  the  calculated  values  for  the  groups,  pl9  p2,  etc.,  the  as- 
signed values  for  the  ultimate  units,  m  the  number  of  groups,  n 
the  number  of  units  in  each  group,  p  the  average  value  of  all 
the  ultimate  units  and  <rp  their  standard  deviation  ;  we  may,  by 
substituting  the  given  values,  derive  the  coefficient  of  correlation. 

{[1,080  -  280]  ^-  4[4(4  -  1)]}  -  42 


L22472 

16.6667  -  16       0.6667 
-T1999-      =L4999 


.  . 
= 


The  magnitude  of  the  coefficient  obtained  may  be  influenced 
by  the  size  of  the  ultimate  and  group  units,  the  nature  of  the 
character  measured,  and  the  variety  or  strain  grown. 

The  above-outlined  method  is  especially  useful  where  it  is 
desirable  to  determine  the  relative  heterogeneity  of  several 
fields.  The  application  of  this  test  for  uniformity  to  a  field 
that  is  being  used  for  experimental  work  would,  in  many  cases, 
prevent  the  use  of  the  field  for  breeding  operations  for  at  least 
a  year. 

Estimating  Soil  Heterogeneity  by  Means  of  Checks.  —  Check 
plots  are  often  used  in  determining  the  comparative  soil  variability 
of  fields  that  are  being  used  for  plot  studies.  This  is  done  by  the 
calculation  of  statistical  constants.  When  used  for  this  purpose 
checks  should  be  systematically  placed  over  the  entire  experi- 
mental area.  The  number  should  be  large  in  order  that  an 
approach  to  a  normal  frequency  distribution  may  be  obtained, 
and  systematic  distribution  should  be  followed  in  order  to  insure 
a  representative  random  sample.  Comparison  of  soils  should  be 
made  in  the  same  year  and  by  the  use  of  the  same  strain  as  the 
check.  In  general,  the  greater  the  degree  of  soil  heterogeneity 
the  greater  will  be  the  calculated  standard  deviation,  coefficient 
of  variability,  and  probable  error. 

Use  of  Checks  in  Correcting  Yields.  —  Aside  from  the  use 
to  indicate  soil  variation,  checks  plots  have  often  been  used 
to  make  direct  corrections  for  yield.  Table  IX,  taken  from 
Wood  and  Stratton  (1910),  illustrates  a  simple  use  of  checks 

±  0.6745  (1  -  r2) 


1  P.  E.  coefficient  of  correlation  = 


Vn 

±  0.6745  ( 1  -  0.4442) 

Vie          =  ±0'135' 


54 


BREEDING  CROP  PLANTS 


for  the  purpose  of  correcting  yields  where  there  is  a  tendency 
to  vary  in  one  direction  across  a  field. 

TABLE  IX. — DIRECT  CORRECTION  FOR  YIELD  WHERE  VARIATION  is  IN  ONE 
DIRECTION  ACROSS  A  FIELD 


Yield  of  Mg-acrc 
plots,  Ib. 

Correction, 
Ib. 

Corrected  yields, 
Ib. 

2,537 

-12  X  25 

2,237 

2,515 

-11  X  25 

2,240 

Mean  2,640  

2,866 

-10  X  25 

2,616 

'   2,648 

-  9  X  25 

2,423 

2,636 

-  8  X  25 

2,4  6 

2,581 

-  7  X  25 

2,406 

2,814 

-  6  X  25 

2,664 

2,944 

-  5  X  25 

2,819 

Difference 

2,748 

-  4  X  25 

2,648 

between  means 

2,593 

-  3  X  25 

2,518 

500  Ih. 

2,567 

-  2  X  25 

2,517 

2,357 

-   1  X  25 

2,332 

2,415 

0  X  25 

2,415 

Correction 

2,424 

4-  1  X  25 

2,449 

from  plot  to 

2,423 

+  2  X  25 

2,473 

plot  «o%o 

2,399 

+  3  X  25 

2,474 

=  25  Ib. 

2,272 

+  4  X  25 

2,372 

2,374 

+  5  X  25 

2,499 

2,123 

+  6  X  25 

2,273 

2,273 

+  7  X  25 

2,448 

2,117 

+  8  X  25 

2,317 

2,001 

+  9  X  25 

2,226 

Mean  2,140  

2,115 

4-10  X  25 

2,365 

2,246 

+  11  X  25 

2,521 

2,222 

+12  X  25 

2,522 

P.E.  ±  7  per  cent! 

P.E.  +  4  per  cent. 

In  the  second  column  of  Table  IX  the  actual  yields  are  given 
of  25  contiguous  J^g-acre  plots  across  a  field.  The  figures  show 
a  more  or  less  gradual  decrease,  reading  from  top  to  bottom. 

There  is  a  difference  of  500  Ib.  between  the  average  yield  of 
the  first  five  plots  and  the  average  yield  of  the  last  five  plots,  or  an 
average  difference  from  plot  to  plot  of  25  Ib.  This  correction 
is  applied  by  adding  to  those  on  one  side  and  subtracting  from 
those  on  the  other  side  of  the  centrally  located  plot.  The  amount 
added  or  substracted  depends  on  the  distance  from  the  center, 


FIELD  PLOT  TECHNIC  55 

i.e.)  a  progressive  difference  of  25  Ib.  for  each  plot  in  either  direc- 
tion from  the  central  one.  The  corrected  yields  are  found  in 
the  last  column  of  the  table.  Note  that  the  probable  error  is 
3  per  cent,  less  in  the  corrected  than  in  the  unconnected  yields. 

The  method  outlined  above  may  be  used  only  where  there  are 
a  comparatively  large  number  of  similarly  treated  plots  and 
where  the  increase  or  decrease  in  yield  across  a  field  is  fairly 
consistent.  If  check  plots  are  grown  every  third  to  fifth  plot 
as  they  frequently  are,  a  direct  correction  for  yield  is  sometimes 
made  as  follows : 

DIAGRAM  ILLUSTRATING  DISTRIBUTION  OF  CHECKS 


c 

1 

2 

3 

Ci             4 

5 

6 

C2 

Suppose  every  fourth  plot  is  a  check.  The  productivity  of 
each  intervening  plot  is  estimated  on  the  basis  of  the  yields  of 
the  two  nearest  checks.  For  instance,  the  true  productivity 
for  plot  one  equals  %C  +  Y^\\  for  plot  two  equals  ^C  +  %C\] 
for  plot  three  equals  Y±C  +  %Ci;  etc.  For  example,  by  this 
method  the  yielding  value  of  plot  six  could  be  obtained.  The 
corrected  yield  could  then  be  obtained  by  the  following  pro- 
portion: Average  yield  of  all  check  plots:  yielding  value  of  plot 
six  =  the  actual  yield  obtained  from  plot  six:  X.  In  a  similar 
way  corrected  yields  could  then  be  obtained  for  all  plots  in  the 
test. 

Use  of  Checks  as  a  Probable  Error  of  the  Experiment. — Other 
methods  of  using  the  checks  as  direct  corrections  for  yield  have 
been  employed,  but  the  tendency  in  present-day  field  investi- 
gations is  away  from  the  use  of  checks  for  this  purpose  (especially 
where  yield  is  being  studied).  They  are,  however,  very  valuable 
indices  of  soil  variation,  thus  giving  an  approximate  measurement 
of  reliability  for  the  particular  experiment.  To  illustrate  the 
use  of  checks  in  this  way,  suppose  in  a  certain  experiment  there 
were  50  systematically  distributed  checks  grown  and  that  the 
computed  probable  error  of  a  single  check  plot  (standard  devia- 
tion X  0.6745)  was  4  bu.  Suppose  each  variety  or  strain  being 
investigated  for  yield  is  replicated  three  times,  making  four 
plots  in  all.  The  probable  error  of  the  average  yield  of  these 
four  plots  would  be  equal  to  the  probable  error  of  a  single  check, 
4  bu.  divided  by  the  square  root  of  the  number  of  plots,  or  4. 
This  gives  2  bu.  as  the  probable  error  of  the  average  yield  of 


56 


BREEDING  CROP  PLANTS 


four  plots.  Using  this  figure  as  a  basis,  a  direct  comparison 
may  be  made  between  the  average  yield  of  any  two  strains  in 
terms  of  their  probable  errors.  To  carry  our  hypothetical 
problem  still  further,  suppose  the  average  yield  of  four  plots  of 
strain  A  was  20  bu.  while  that  of  strain  B  was  26  bu.  As  the 
probable  error  of  each  average  is  2  bu.,  and  the  probable  error  of 
a  difference  is  equal  to  the  square  root  of  the  sum  of  the  squares 
of  the  probable  errors  of  the  two  quantities,  we  would  have 

26  ±  2 

20  +  2 

6  +      V^M  or  6  ±  2.8 
The  difference  is  only  a  little  more  than  two  times  its  probable 

TABLE  X. — PROBABILITY  OF  OCCURRENCE  OF  STATISTICAL  DEVIATIONS  OF 
DIFFERENT  MAGNITUDES  RELATIVE  TO  THE  PROBABLE  ERROR 


Deviation 
divided  by 
P.E. 

Probable  oc- 
currence of  a 
deviation  as 
great  as  or 
greater  than 
designated 
one  in  100 

Odds  against 
the  occurrence 
of  a  deviation 
as  great  as  or 
greater  than 
the  designated 

Deviation 
divided  by 
P.E. 

Probable  oc- 
currence of  a 
deviation  as 
great  as  or 
greater  than 
designated 
one  in  100 

Odds  against  the  oc- 
currence of  a  deviation 
as  great  as  or  greater 
than  the  designated 
one 

trials 

one 

trials 

1.0 

50.00 

1.00  to 

3.5 

1.82 

53.  95  to  1 

1.1 

45.81 

1.18  to 

3.0 

1.52 

64.  79  to  1 

1.2 

41.83 

1.39  to 

3.7 

1.26 

78.37  to  1 

1.3 

38.06 

1.63  to 

3.8 

1.04 

95  .  15  to  1 

1.4 

34.50 

1.90  to 

3.9 

0.853 

116.  23  to  1 

1.5 

31.17 

2.21  to 

4.0 

0.698 

142.  26  to  1 

1.6 

28.05 

2.  57  to 

4.1 

0.569 

174.  75  to  1 

1.7 

25.15 

2.  98  to 

4.2 

0.461 

215.  92  to  1 

1.8 

22.47 

3.  45  to         ! 

4.3 

0.373 

267  .  10  to  1 

1.9 

20.00 

4.  00  to 

4  .4 

0.300 

332.  33  to  1 

2.0 

17.73 

4  .  64  to  1 

4.5 

0.240 

415.  67  to  1 

2.1 

15.67 

5.38  to  1 

4.6 

0.192 

519.  83  to  1 

22 

13.78 

6.26  to  1 

4.7 

0.152 

656.  89  to  1 

2.3 

12.08 

7.  28  to  1 

4.8 

0.121 

825.  45  to  1 

2.4 

10.55 

8.48  to  1 

4.9 

0.095 

1,051.  63  to  1 

2.5 

9.18 

9.  89  to  1 

5.0 

0.074 

1,350.  35  to  1 

2.6 

7.95 

11.  58  to  1 

6.0 

0.0052 

19,230.  00  to  1 

2.7 

6.86 

13.58  to  1 

7.0 

0.00023 

434,782.  00  to  1 

2.8 

5.90 

15.  95  to  1      j 

8.0 

0.000000068 

1,470,588,  234.  00  to  1 

2.9 

5.05 

18.80  to  1 

3.0 

4.30 

22.  26  to  1 

3.1 

3.65 

26.  40  to  1 

3.2 

3.09 

31.  36  to  1 

* 

3.3 

2.60 

37.  46  to  1 

3.4 

2.18 

44.  87  to  1 

FIELD  PLOT  TECHNIC 


57 


error  and,  therefore,  under  the  assumed  conditions  of  the  experi- 
ment, of  little  significance,  as  is  indicated  by  Table  X  taken  from 
Pearl  and  Miner  (1914). 

Use  of  Probable  Error  in  Eliminating  Strains. — The  probable 
error  obtained  by  means  of  the  checks  may  also  aid  in  selecting 
an  elimination  value  below  which  varieties  or  strains  may  be  dis- 
carded without  danger  of  throwing  away  a  valuable  one.  This 
figure  is  necessarily  more  or  less  arbitrary  and  will  depend  upon 
the  desired  degree  of  accuracy.  The  magnitude  of  the  figure 
which  is  multiplied  by  the  probable  error  will  also  depend  some- 
what upon  the  desired  amount  of  elimination.  The  method 
used  at  the  Minnesota  Station  is  to  subtract  the  product  of  three 
times  the  probable  error  for  the  method  of  test  multiplied  by 
\/2  from  the  highest  or  one  of  the  higher  yielding  strains.  The 
difference  gives  a  figure  below  which  it  is  considered  safe  to  dis- 
card without  danger  of  eliminating  a  high  yielding  strain.  If 
the  yield  of  a  strain  falls  below  the  elimination  figure  for  two  or 
three  years,  it  is  discarded  from  further  trials. 

The  Pairing  Method  of  Securing  a  Probable  Error. — Under 
certain  conditions  it  is  impracticable  to  devote  so  large  a  share 
of  the  experimental  field  to  check  plots.  Wood  and  Stratton 
(1911)  have  suggested  a  means  of  securing  a  reliable  probable 
error  without  the  aid  of  checks.  Briefly,  their  method  consists 
of  systematically  pairing  similarly  treated  plots  and  finding  their 
mean  yields.  The  deviation  of  this  mean  from  the  yield  of  the 
original  plots  is  expressed  in  percentage  of  the  mean.  The  fol- 
lowing illustrates  the  procedure: 


Plot      arrange- 

ment   

A' 

B' 

C' 

etc. 

A" 

B" 

C" 

etc. 

A"  ' 

B"  ' 

C"' 

etc. 

Yield   per   acre 

20 

22 

24 

etc. 

21 

23 

25 

etc. 

24 

22 

23 

etc. 

Now  if  all  A  plots  are  similarly  treated,  A'  would  be  paired 
with  A"  and  A"  with  A'",  etc. 

In  this  method  the  probable  error  is  expressed  in  percentage  of 
the  mean.  If  the  number  of  pairs  is  sufficiently  great  the  devia- 
tions +  and  —  will  yield  a  normal  frequency  curve.  As  in  the 
method  of  determining  the  probable  error  by  means  of  checks, 
it  is  desirable  to  have  a  large  enough  number  of  variants  to  secure 
at  least  an  approach  to  the  normal  frequency  distribution. 


58 


BREEDING  CROP  PLANTS 
TABLE  XI. — THE  PAIRING  METHOD 


Plot 

Yield 

Mean 

Deviation 

Deviation  in 
percentage  of 
mean 

Deviation  in 
percentage  of 
mean  squared 

A'  :... 

201 

20.5 

M 

2.4 

5.76 

A" 

2l| 

A"     

211 

22.5 

1.5 

6.7 

44.89 

A"'             

?4\ 

etc  

Total  
Average  

86 
21.5 

2.0 
1.0 

9.1 
4.6 

50.65      . 
25.33 

After  the  deviations  have  been  converted  into  percentages  of  the 
mean,  their  sum  is  divided  by  n  where  n  is  the  number  of  pairs. 
By  multiplying  the  average  yield  of  all  plots  by  this  percentage,  a 
probable  error  for  yield  of  a  single  plot  may  be  obtained. 

Wood  and  Stratton  (1910)  present  the  following  probable 
errors  obtained  by  the  pairing  method,  based  on  a  large  number  of 
replicated  plots  including  the  different  crops  —  wheat,  barley,  oats, 
mangels,  rutabagas,  potatoes,  and  seed  grasses. 

400  pairs  of  plots,  different  sizes  .........   P.E.  4.2  per  cent. 


45  pairs  of  plots,  each 
52  pairs  of  plots,  each 
29  pairs  of  plots,  each 
200  pairs  of  plots,  each 
75  pairs  of  plots,  each 


acre  ..........  P.E.  3.5  per  cent. 

acre  ..........  P.E.  3.5  per  cent. 

o  acre  .........  P.E.  3.9  per  cent. 

acre  .......  .  .  P.E.  4.6  per  cent. 

o  acre  .........  P.E.  3.1  per  cent. 


In  applying  the  method  of  Wood  and  Stratton  at  the  Min- 
nesota Experiment  Station  it  was  found  that  a  slight  modifica- 
tion usually  gave  probable  errors  which  more  nearly  approached 
those  obtained  by  the  use  of  check  plots.  The  modification 
consisted  of  squaring  the  deviations  before  dividing  by  n  and 
extracting  the  square  root  of  the  quotient.  This  calculation 
is  given  in  the  last  column  of  the  preceding  table. 

Replication  and  Its  Value.  —  It  has  been  found  that  systematic 
repetition  of  the  plots  reduces  the  probable  error  and  hence 
increases  the  significance  of  the  results.  The  number  of  replica- 
tions necessary  in  order  to  make  reliable  comparisons  is  some- 
what dependent  on  the  kind  of  crop  but  to  a  greater  extent  on  soil 
heterogeneity.  If  it  were  desired  to  establish  a  significant  dif- 
ference of  as  little  as  2  bu.  between  varieties,  more  replications 
would  be  needed  than  if  a  significant  difference  of  4  bu.  was 
accepted  as  satisfactory.  Several  investigations  have  been  re- 


FIELD  PLOT   TECHNIC 


59 


ported  which  for  the  particular  condition  of  the  experiment  show 
the  number  of  replications  desirable. 

Mercer  and  Hall  (1911),  of  England,  recommend  the  use  of 
five  systematically  distributed  plots  of  J^Q  acre  each.  Mont- 
gomery (1913),  in  his  work  at  Nebraska,  found  that  16  ft.  rows 
gave  best  results  when  repeated  from  10  to  20  times.  At  the 
Cornell  Experiment  Station,  when  a  careful  yield  test  is  desired 
each  strain  is  grown  in  10  distributed  rod  rows. 

In  the  plant-breeding  nursery  of  the  Minnesota  Experiment 
Station  the  practice  is  followed  of  growing  each  strain  in  a  plot 
consisting  of  three  rod  rows.  The  plots  are  replicated  three 
times,  making  four  plots  in  all.  The  central  rows  only  are 
harvested.  Table  XII  taken  from  Hayes  and  Arny  (1917) 
shows  the  effect  of  replication  based  on  the  yield  of  the  central 
rows  of  the  wheat  checks  grown  in  1916. 

TABLE  XII. — VALUE  OF  REPLICATION  BASED  ON  72  CENTRAL  Rows  OP 

THREE-ROW  PLOTS  OF  TURKEY  WINTER  WHEAT  (MINN.  529) 

GROWN  IN  THE  PLANT  BREEDING  NURSERY 


Number  of  replications 

Number  of 
variables 

Mean  yield 
per  acre 

Standard 
deviation 

None               .          .  .                

72 

27.5  +  0.4 

4.65  +  0.26 

One  (average  of  2  plots)  

36 

27.6  +  0.3 

2.98  +  0.24 

Two  (average  of  3  plots)  
Three  (average  of  4  plots)  
Five  (average  of  6  plots)  
Eleven  (average  of  12  plots)  

24 

18 
12 
6 

27.4±0.3 
27.6±0.2 
27.3+0.4 
27.3+0.3 

2.51+0.24 
1.49±0.17 
2.01+0.28 
1.21±0.24 

While  the  standard  deviations  do  not  decrease  according  to 
theoretical  expectation  they  do  show  a  marked  decrease  up  to  and 
including  three  replications.  The  table  shows  that  variability 
is  rapidly  diminished  by  replication  up  to  a  certain  number. 
In  general,  beyond  this  point  it  is  questionable  whether  the 
relatively  small  gain  in  accuracy  warrants  the  additional  work. 
The  results  obtained  at  the  Minnesota  Station  indicate  that 
plots  of  three  rod  rows  each  or  J^o-acre  plots  sown  with  the  or- 
dinary grain  drill,  give  about  as  accurate  a  comparison  for  yield 
when  replicated  three  times  as  when  replicated  eight  times. 

The  manner  of  making  replications  is  another  factor  to  be  con- 
sidered. If  the  experimental  plots  are  all  planted  in  a  single 


60 


BREEDING  CROP  PLANTS 


series  then  replication  becomes  a  matter  of  systematic  repetition 
as  is  shown  by  the  following  diagram  in  which  each  different  letter 
represents  a  distinct  strain. 


A  METHOD  OP  REPLICATION 


ABCDEFGHABCDE.FGHABCDEFGHABCDEFGH 


As  a  rule  the  experimental  plots  cannot  all  be  placed  in  the 
same  series.  It  is  often  necessary  to  make  alterations  from  a 
mere  systematic  repetition  in  order  to  secure  a  representative  dis- 
tribution of  the  strains.  The  two  following  diagrams  illustrate 
a  correct  and  an  incorrect  manner  of  replication: 


CORRECT  MANNER  OF  REPLICATION 


INCORRECT  MANNER  OF  REPLICATION 


A 

J 

G 

D 

B 

K 

H 

E 

C 

L 

I 

F 

D 

A 

J 

G 

E 

B 

K 

H 

F 

C 

L 

I 

G 

D 

.  A 

J 

II 

E 

B 

K 

I 

F 

C 

L 

J 

G 

D 

A 

K 

H 

E 

B 

L 

I            F            C 

A 

A 

A 

A 

B 

B 

B 

B 

C 

C 

C 

C 

D 

D 

D 

D 

E 

E 

E 

E 

F 

F 

F 

F 

G 

G 

G 

G 

H 

H 

H 

H 

I 

I 

I 

I 

J 

J 

J 

J 

K 

K 

K 

K 

L 

L 

L 

L 

Size  of  Plot. — The  number  of  replications  required  to  secure 
a  given  degree  of  accuracy  is  somewhat  dependent  on  the  area 
of  the  plot.  Mercer  and  Hall  (1911)  found  that  variability  is 
diminished  with  increased  size  of  plot  up  to  J4o  acre.  Plots  of 
larger  area  do  not  show  the  same  relative  reduction  in  variability. 
Fig.  13  presents  graphically  their  results  with  wheat. 

Montgomery  (1913)  also  finds  that  increased  size  of  plot, 
up  to  a  certain  limit,  rapidly  decreases  variability.  In  plant- 
breeding  work  where  very  numerous  strains  are  compared,  the 


FIELD  PLOT  TECHNIC 


61 


size  of  plot  is  necessarily  limited  by  available  space  and  some- 
times by  amount  of  seed.  Some  form  of  row  planting  is  usually 
followed.  These  rows  are  planted,  cultivated,  and  harvested 
by  hand  and  frequently  show  as  low  probable  errors  as  those 
obtained  from  J^o-acre  field  plots. 


r 

<o   4 

3 


1  1 .      1_ 

500250  125 


i-  Size  of  Plot 


FIG.   13. — Actual  and  theoretical  reduction  in  standard  deviation  due  to  increase 

in  size  of  plot. 

Shape  of  Plot  and  Border  Effect. — Often  plants  growing  along 
the  side  or  end  of  the  plot  are  more  thrifty  and  vigorous  than 
those  growing  in  the  interior.  When  plots  consist  of  single  rows, 
the  plants  at  the  extremities  near  the  alleys  or  pathways  appear 
superior  to  those  growing  farther  in. 

Mercer  and  Hall  (1911)  cut  up  a  bulk  field  into  plots  of  equal 
area  but  different  in  shape  (approximately  20  by  12  yd.  and  50  by 
5  yd.)  and  therefore  without  border  effect.  No  significant  differ- 
ence in  comparative  variability  was  found  between  the  two 
shapes.  Barber  (1914)  found  that  where  cultivated  pathways 
surrounded  plots,  the  plants  along  the  margins  were  more  pro- 
ductive than  those  within  the  plot. 

Table  XIII  presents  data  collected  by  Arny  and  Hayes  (1918). 
The  plots  were  seeded  with  a  grain  drill,  the  drill  rows  being  6  in. 
apart.  Eighteen-inch  alleys  separated  the  plots,  and  there  was 
a  roadway  along  each  end.  In  length  the  plots  were  trimmed 
to  132  feet.  In  breadth  they  were  17  drill  rows,  each  6  inches 
apart.  Each  of  the  two  outside  border  rows  was  harvested  sepa- 
rately and  the  yield  compared  with  the  yields  obtained  from  the 


62 


BREEDING  CROP  PLANTS 


central  rows.     The  plants  on  each  end  of  the  plots  to  a  depth  of 
at  least  a  foot  were  cut  and  discarded. 

TABLE  XIII. — COMPARISON  OF  AVERAGE  YIELD  OF  OATS,  WHEAT  AND  BARLEY 

HARVESTED  FROM  BORDER  Rows  AND  CENTRAL  Rows 

PLOTS  132  BY  8.5  FT. 


Source 

Oats 

Wheat 

Barley 

No.  of 
plots 

Yld. 
per 
acre, 
bu. 

No.  of 
plots 

Yld. 
per 
acre, 
bu. 

No.  of 
plots 

Yld. 
per 
acre, 
bu. 

Outside  border  rows  

44 

132.0 

20 

55.0 

16 

97.7 

Inside  border  rows  

44 

88.0 

20 

41.0 

16 

64.5 

Central  13  rows  

44 

71.4 

20 

27.5 

16 

42.9 

It  is  clear  from  Table  XIII  that  border  effect  may  profoundly 
influence  yield.  Long,  narrow  plots  have  a  larger  proportion  of 
their  area  in  border  than  those  which  more  nearly  approach  a 
square.  This  would  seem  to  indicate  that  square  plots  should 
be  given  preference  over  oblong  ones  unless  the  borders  are 
discarded.  As  a  matter  of  fact,  most  workers  use  long,  narrow 
plots  because  of  the  greater  ease  with  which  they  may  be  seeded 
and  harvested  with  machinery.  Furthermore,  border  effect 
may  be  entirely  removed  by  discarding  the  borders  and  ends 
of  the  plot. 

The  removal  of  borders  becomes  still  more  desirable  when  the 
fact  is  considered  that  different  strains  and  varieties  may  react 
unequally  to  borders  or  ends.  Evidence  has  been  accumulated 
which  shows  that  some  strains  utilize  the  border  to  a  greater 
degree  than  others.  Obviously  those  strains  which  gain  least 
from  the  alley  space  will  not  be  given  a  fair  trial  unless  border 
rows  are  discarded  from  all  the  plots  in  the  experiment.  Con- 
versely, those  which  have  the  greater  ability  to  use  the  border 
may  be  given  a  higher  rating  than  they  deserve  unless  the  bor- 
ders are  removed. 

It  would  seem  from  the  evidence  presented  that  it  is  highly 
desirable  to  discard  ends  and  borders  to  a  depth  of  at  least  a  foot 
in  the  case  of  rectangular  plots  and  a  foot  at  each  end  in  the  case 
of  rod  rows  growing  side  by  side. 

Competition  as  a  Factor  in  Plot  Variability. — Competition 
between  nearby  strains,  particularly  under  certain  experimental 
conditions,  may  seriously  influence  results.  A  tall  variety  may 


FIELD  PLOT  TECH  NIC 


63 


hamper  a  shorter  one,  or  a  vigorous  grower  may  inhibit  one  that 
grows  more  slowly.  The  existence  of  competition  between  ad- 
jacent strains  or  varieties  has  been  definitely  proved  at  several 
experiment  stations.  The  work  of  Kiesselbach  (1918)  at  the 
Nebraska  Station  is  particularly  illuminating  on  this  point. 
Kiesselbach  compared  competition  between  adjacent  single  row 
plots  and  adjacent  plots  each  consisting  of  from  three  to  five  rows. 
The  yield  of  border  rows  was  in  some  instances  included  in 
the  yield  of  the  blocks.  His  results  are  summarized  in  Table. 
XIV. 


TABLE  XIV. — SUMMARY  OF  RELATIVE  GRAIN  YIELDS  OF  VARIETIES  TESTED 

IN  SINGLE-ROW  PLOTS  AND  ALSO  IN  BLOCKS  CONTAINING 

SEVERAL  Rows 


Varieties  compared  in  alternating  rows 
and  in  alternating  blocks 

Year 
of  test 

Ratio  of  variety  No.  1  to  variety  No.  2 

Alternating 
rows 

Alternating 
blocks 

Competing 
in  same  hill 

Turkey  Red  (1)  and  Big  Frame  (2) 
winter  wheat  

1913 
1914 
1913 

1914 
1913 
1914 

1913 
1914 
1912 
1914 
1914 
1916 

100:107 
100:85 
100:107 

100:63 
100:130 
100:139 

100:82 
100:89 
100:66 
100:38 
100:90 
100:31 

100:97 
100:97 
100:107 

100:85 
100:112 
100:101 

100:77 
100:93 
100:85 
100:53 
100:98 
100:37 

100:47 
100:26 
100:99 
100:21 

Turkey  Red  (1)  and  Big  Frame  (2) 
winter  wheat  

Turkey  Red  (1)  and  Nebraska  No. 
28  (2)  winter  wheat  

Turkey  Red  (1)  and  Nebraska  No. 
28  (2)  winter  wheat  

Kherson  (1)  and  Burt  (2)  oats  .  .  . 
Kherson  (1)  and  Burt  (2)  oats.  .  .  . 
Kherson   (1)  and  Swedish  Select 
(2)  oats 

Kherson  (1)  and  Swedish  Select 
(2)  oats                            

Hogue's    (1)    and    Pride    of    the 
North  (2)  corn     

Hogue's    (1)    and    Pride    of    the 
North  (2)  corn  

Hogue's  (1)  and  University  No.  3 
(2)  corn 

Crossbred  Rogue's  (1)  and  inbred 
Hogue's  (2)  corn                

A  comparison  of  the  columns  in  Table  XIV  headed  alternat- 
ing rows  and  alternating  blocks  shows  strikingly  the  effects  of 
competition.  In  almost  every  case  the  varieties  grown  in  alter- 


64  BREEDING  CROP  PLANTS 

nating  rows  show  a  greater  difference  than  the  same  varieties 
grown  in  alternating  blocks.  Perhaps  the  extreme  effect  of  com- 
petition is  shown  by  different  varieties  of  corn  grown  in  the  same 
hill. 

In  the  plots  consisting  of  three  rows  each,  grown  at  the  Minne- 
sota Experiment  Station  in  1916,  a  study  of  competition  was 
made.  When  varieties  of  different  heights  were  grown  in  adja- 
cent plots  a  considerable  effect  was  obtained  in  the  yields  of  border 
rows  in  the  barley  and  winter  wheat  nurseries.  The  effect  of 
competition  has  been  observed  at  other  experiment  stations  and 
various  ways  of  overcoming  its  possible  vitiating  influence  have 
been  suggested. 

One  of  the  easiest  and  most  effective  means  of  eliminating  this 
source  of  error  is  by  the  use  of  sufficiently  wide  borders  which  are 
discarded  at  harvest.  In  the  case  of  plots,  consisting  of  a  single 
row,  it  is  possible  to  make  the  planting  plan  in  such  a  way  as  to 
minimize  effects  of  competition.  The  rows  should  be  laid  out 
north  and  south  and  the  varieties  and  strains  most  nearly  alike 
in  habits  of  growth  should  appear  side  by  side.  At  best  this 
method  can  do  no  more  than  decrease  the  error  due  to  compe- 
tition, while  the  elimination  of  effective  borders  overcomes  com- 
petition. The  use  of  borders  necessitates  a  larger  experimental 
area  and  is  somewhat  more  expensive  for  a  given  number  of 
trials. 

CLIMATIC  VARIATIONS 

One  other  disturbing  factor  to  be  considered  in  conducting  plot 
tests  is  variation  induced  by  weather  conditions.  Its  presence 
is  so  obvious  to  any  one  who  has  worked  with  growing  crops  that 
further  comment  is  hardly  necessary.  In  a  year  of  deficient  rain- 
fall the  varieties  best  qualified  to  subsist  under  a  minimum  water 
supply  will  yield  most.  Some  seasons  are  better  for  the  growth 
of  early  maturing  varieties  than  for  late  ones.  An  epidemic  of  a 
plant  disease  like  rust  may  be  fostered  or  hampered  by  weather 
conditions.  The  question  arises,  how  may  errors  due  to  this 
source  be  overcome?  Conducting  an  experiment  over  a  period 
of  years  is  the  only  effective  means  at  the  disposal  of  the  investi- 
gator. The  strain  which  fluctuates  the  least  from  year  to  year 
and  also  gives  a  high  average  performance  is  most  valuable  for 
the  farmer. 


FIELD  PLOT  TECH  NIC  65 

SUMMARY  OF  FIELD  PLOT   TECHNIC 

Following  is  a  brief  summary  of  the  more  important  factors 
which  assist  in  obtaining  reliable  plot  results. 

1.  Soil  heterogeneity  exists  in  varying  degrees,  hence  uniform 
plots  should  be  selected  for  the  field  experiments.     To  aid  in 
determining    the    comparative    uniformity    of    different    fields, 
Harris'  method  or  the  check  plot  method  may  be  used. 

2.  If  the  field  varies  uniformly  from  one  side  to  the  other, 
check  plots  may  be  used  to  correct  yields.     In  general,  the  use  of 
checks  to  correct  yields  is  undesirable. 

3.  The  yield  of  check  plots  may  be  used  to  determine  the  prob- 
able error  of  the  method  of  work.     They  should  be  placed  system- 
atically throughout  the  experimental  plots  and  the  number  should 
be  sufficiently  large  to  approach  a  normal  frequency  distribution. 

4.  Probable  errors  may  be  used  to  determine  whether  the  ob- 
tained differences  between  strains  are  significant  and  thus  aid  in 
eliminating  the  significantly  lower  yielders. 

5.  The  probable  error  of  an  experiment  may  be  determined 
by  the  pairing  method  suggested  by  Wood  and  Stratton.     It  is 
comparable  to  the  one  based  on  the  checks  and  may  be  used  in 
the  same  way. 

6.  The  probable  error  of  an  experiment  may  be  reduced  most 
effectively  by   plot  replication.     Replication  up   to  a   certain 
number  rapidly  reduces  the  probable  error,  beyond  that  number 
additional  replications  do  not  proportionately  decrease  it.     The 
number  of  replications  will  depend  considerably  on  the  character 
of  the  soil  and  somewhat  on  the  size  of  the  plots.     On  fairly  uni- 
form land  three  replications  have  been  found  satisfactory  for 
general  breeding  studies. 

7.  Oblong  plots  sown  with  an  ordinary  grain  drill  give  reliable 
results  when  their  area  is  approximately  J4o  acre  each. 

8.  Plants  growing  on  the  border  of  a  plot  adjacent  to  an  alley 
or  roadway  are  usually  superior  to  those  growing  within  the  plot, 
hence,  if  it  is  desired  to  secure  yields  comparable  with  those  which 
would  be  secured  under  field  conditions,  the  border  plants  must 
be  discarded.     The  border  should  be  removed  to  a  depth  of  at 
least  a  foot.     Different  varieties  and  strains  may  have  unequal 
ability  to  utilize  the  free  space  along  the  pathways  between  plots 
and  consequently  a  second  reason  arises  for  discarding  the  border. 

9.  Competition  exists  between  nearby  varieties  and  strains, 

5 


66  BREEDING  CROP  PLANTS 

The  grouping  of  varieties  and  strains  so  that  those  of  similar 
habits  of  growth  appear  side  by  side  removes  to  a  considerable 
degree  the  evil  effects  of  competition.  The  most  effective  means 
of  overcoming  competition  is  by  the  use  of  sufficiently  wide 
borders  which  are  discarded  at  harvest. 

10.  Results  of  field  tests  vary  from  year  to  year  because  of 
changing  weather  conditions,  and  for  this  reason  it  is  necessary 
to  extend  a  test  over  a  period  of  several  years.  For  varietal  trials 
a  minimum  of  three  years  is  recommended. 


CHAPTER  V 
CONTROLLING  POLLINATION 

Methods  of  controlling  pollination  have  received  consider- 
able attention.  Protecting  self-fertilized  plants  from  occasional 
natural  crosses  would  seem  to  be  a  necessity  in  careful  studies 
of  heredity.  The  lack  of  technic  of  crossing  may  be  a  cause  of 
failure  to  improve  a  particular  crop.  This  entire  field  is  one  in 
which  actual  practice  is  needed  before  the  worker  can  hope  to 
accomplish  best  results.  A  few  general  principles  will  be  given. 

Selfing  Plants  Artificially. — Certain  methods  have  already 
been  worked  out  for  particular  crops.  As  an  example,  in  the 
tobacco  crop  artificially  self-fertilized  seed  may  easily  and  cheaply 
be  produced.  The  practical  grower  can  well  afford  to  save 
his  seed  by  this  practice.  Before  any  of  the  blossoms  have 
opened,  the  terminal  inflorescence  should  be  covered  with  a 
manila  paper  bag.  The  12-lb.  size  has  been  found  satisfactory 
for  this  purpose.  If  a  few  flowers  have  already  been  pollinated 
these  may  be  removed  before  bagging.  After  a  week  or  10  days 
has  elapsed,  the  bag  should  be  taken  off  and  all  flowers  except 
from  50  to  60  removed  and  the  dead  corollas  shaken  off.  After 
sufficient  flowers  have  been  fertilized  the  bag  may  be  removed,  as 
the  seed  will  mature  somewhat  more  rapidly  than  when  enclosed. 

Self-pollination  of  the  tomato  may  be  accomplished  in  very 
much  the  same  manner  as  with  tobacco.  Small-sized  bags  are 
needed.  In  this  case  it  is  necessary  to  jar  the  flowering  branches 
upon  which  the  bags  are  placed  as  the  tomato  does  not  set  seed 
freely  unless  some  such  practice  is  followed. 

f  Artificial  self-pollination  in  corn  is  very  easy.  The  ear  and 
tassel  may  each  be  covered  with  a  12-lb.  manila  paper  bag. 
It  is  necessary  to  cover  the  ear  before  any  of  the  silks  show. 
Foreign  pollen  accidentally  enclosed  with  the  tassel  will  not 
function  after  a  period  of  more  than  two  days.  Approximately 
two  to  five  days  after  the  ear  has  been  bagged  the  silks  will  have 
grown  out  and  will  be  ready  for  pollination.  The  most  favorable 
time  for  pollination  is  when  the  silks  are  2  to  3  in.  long,  although 
the  silks  are  receptive  when  much  longer. 

67 


68  BREEDING  CROP  PLANTS 

Two  men  may  well  work  together  in  pollination.  One  unties 
the  ear  bag  and  the  other  shakes  the  dead  anthers  from  the 
tassel  bag  and  pours  the  pollen  over  the  silk.  Care  is  needed 
in  performing  this  operation  to  prevent  cross-  or  uncontrolled 
pollination.  In  producing  biotypes  by  self-fertilization  the 
occasional  cross  may  easily  be  rogued  out  as  the  crossed  plant 
will  plainly  be  seen  the  following  year  because  of  its  vigor  and 
other  characters.  Some  workers  prefer  transparent  paper  bags 
.which  allow  the  development  of  the  silks  to  be  noted  without 
removing  the  bag  from  the  ear,  and  thus  save  unnecessary  work. 

Hard  showers  or  long  continued  rains  seriously  interfere  with 
the  artificial  pollination  of  corn,  as  the  tassel  bag  becomes  wet 
and  makes  the  handling  of  the  pollen  difficult.  A  desirable 
method  is  to  remove  the  tassel  bags  after  each  rain  and  put  on 
new  ones.  As  a  number  of  days  elapse  from  the  time  the  first 
pollen  of  the  tassel  matures  until  all  is  mature,  the  method 
of  replacing  tassel  bags  gives  good  results. 

Self-pollination  of  squash  has  been  carried  out  at  the 
Minnesota  Station.  A  little  practice  helps  in  determining  when 
a  flower  is  about  ready  to  open.  The  petals  of  both  staminate 
and  pistillate  flowers  are  prevented  from  opening  by  placing  a 
small  rubber  band  around  each  one.  On  removing  the  band  the 
following  day  the  flower  quickly  opens  if  it  is  ready  for  pollina- 
tion. The  petals  are  then  removed  from  the  staminate  flower 
and  the  anthers  rubbed  over  the  pistil.  The  artificially  pollin- 
ated flower  is  protected  from  cross-pollination  by  placing  a 
rubber  band  around  the  petals.  After  a  few  days  the  petals  of 
the  crossed  flower  abciss  and  at  this  time  the  stigma  has  turned 
brown  and  is  no  longer  receptive.  This  method  was  worked 
out  by  John  Bushnell,  a  graduate  student  in  horticultural  plant 
breeding.  From  a  total  of  600  pollinations  made  under  field 
conditions  in  the  summer  of  1919,  approximately  150  set  fruit. 

Technic  of  Crossing. — A  thorough  knowledge  of  flower  struc- 
ture of  the  species  or  variety  to  be  worked  with  is  essential  before 
crossing  is  undertaken.  It  is  important  to  know  which  flowers 
are  the  most  vigorous  and  which  set  fruit  the  most  freely.  Many 
varieties  of  wheat,  for  example,  produce  several  seeds  per  spike- 
let.  The  outer  florets  of  the  spikelets  in  the  central  part  of  the 
rachis  are  more  vigorous  and  usually  produce  larger  seed.  In 
some  Solanacece  (for  example,  the  petunia)  the  later  flowers 
form  larger,  healthier  seed  than  those  which  first  open  (East, 


CONTROLLING  POLLINATION  69 

1910c).  After  becoming  familiar  with  the  flower  structure  it 
is  important  to  determine  at  what  time  of  day  the  pollen  is 
most  easily  collected  and  for  what  length  of  time  the  stigma  is 
receptive.  Environmental  conditions  modify  the  expression 
of  these  and  other  characters.  However,  some  general  rules 
for  different  groups  of  crops  may  be  given. 

Certain  tools  are  essential  for  the  work  of  pollination.  For 
general  work  these  are  a  small  pair  of  thin,  pointed  scissors;  a 
pair  of  forceps  with  thin,  pointed  blades  which  meet  exactly 
and  which  are  not  too  stiff;  one  or  two  dissecting  needles;  a 
hand  lens;  a  pencil;  and  small  string  tags  for  recording  purposes. 
Other  special  apparatus  is  necessary  for  difficult  crosses. 

Crossing  of  Small  Grains. — The  technic  of  small  grain  crossing 
is  comparatively  simple.  Some  practice,  however,  is  necessary 
in  order  to  gain  proficiency  and  to  obtain  a  fair  percentage  of 
seeds  set.  In  some  of  the  earlier  directions  it  was  stated  (Hays, 
1901)  that  it  was  necessary  to  make  crosses  of  wheat  at  about 
4  o'clock  in  the  morning.  Leighty  and  Hutcheson  (1919)  have 
determined  the  period  in  which  blooming  takes  place  at  Univer- 
sity Farm,  St.  Paul,  Minn.,  and  at  Arlington  Farm,  Rosslyn, 
Va.  The  spikes  were  examined  at  7  a.m.,  12  n.,  and  5  or  6  p.m. 
A  flower  was  considered  as  having  bloomed  when  the  glumes 
had  opened  appreciably.  The  period  from  5  or  6  p.m.,  to  7 
or  8  a.m.  was  referred  to  as  night.  Of  2,977  wheat  flowers  on 
69  spikes,  1,492  bloomed  at  night  and  1,485  bloomed  during  the 
day.  About  half  of  those  which  bloomed  during  the  day  bloomed 
before  noon.  These  figures  are  given  to  correct  the  erroneous 
idea  that  it  is  always  necessary  to  pollinate  wheat  early  in  the 
morning.  Environmental  conditions  may  be  an  important 
factor,  for  Salmon  (1914),  working  in  South  Dakota,  stated 
that  blooming  was  practically  completed  before  7  o'clock  in  the 
morning. 

Leighty  and  Hutcheson  (1919)  show  that  in  wheat  it  is  unsafe 
to  leave  the  spikes  uncovered  after  emasculation.  Seeds  were 
formed  by  507  of  1,240  emasculated,  unprotected  flowers  at 
University  Farm,  Minn,  and  1,103  seeds  were  formed  in  1,324 
flowers  similarly  handled  at  Arlington  Farm,  Va.  while  less  than  1 
per  cent,  of  flowers  emasculated  and  covered  with  paper  bags  set 
seed.  Frear  (1915) ,  working  with  Turkey  winter  wheat,  obtained 
80  per  cent,  seeds  set  on  emasculated,  uncovered  spikes  and  less 
than  1  per  cent,  on  emasculated  covered  spikes. 


70 


BREEDING  CROP  PLANTS 


A  common  practice  used  at  Minnesota  University  Farm  is  to 
emasculate  a  number  of  spikes  one  day  and  make  the  crosses  from 
one  to  four  days  later  at  about  the  time  when  the  flowers  open. 


FIG.  14. — Details  of  wheat  inflorescence. 

Upper  left,  normal  spikes ;  lower  right,  emasculated  spike;  2,  spikelet  natural  size ;  /  and  g, 
flowerless  glumes;  k  and  r,  florets;  3,  a  single  flower  closed  just  after  flowering,  3n;  4A, 
longitudinal  diagram  before  flowering,  x  2.5n,  a  =  anthers,  o  =  ovary,  s  =  stigma,  /  =  filament ; 
4B  =  diagram  after  flowering ;  o  =  transverse  floral  diagram,  Qn,  fg  =  lemma,  p  =  palea,  a  =  an- 
thers, s  =  stigma;  6,  flowerless  glume,  7,  lemma,  8,  palea,  slightly  reduced;  9,  lodicule,  4n;  10, 
cross-section  anther,  26n;  11,  pollen  grains;  12,  ovary  and  stigma  just  prior  to  flowering;  13, 
at  flowering;  and  14,  shortly  after;  15,  16,  17,  the  mature  seed.  (After  Babcock  and  Clausen, 
1918,  after  Hays  and  £oa«.) 


CONTROLLING  POLLINATION  71 

All  but  the  outer  florets  of  eight  of  the  central  spikelets  are  removed. 
The  upper  and  lower  spikelets  are  cut  off  with  shears  and  the 
central  floret  of  each  remaining  spikelet  is  removed  by  grasping 
it  near  the  top  with  the  forceps  and  giving  a  downward  pull. 
The  forceps  are  then  carefully  pushed  between  the  palea  and 
lemma  and  the  flower  opened.  The  three  stamens  are  removed 
in  one  operation,  if  possible.  Care  is  taken  not  to  pinch  the 
anthers  too  tightly  and  break  them  open.  Spikes  are  used  in 
which  the  anthers  are  just  beginning  to  turn  yellow.  Anthers 
from  the  variety  to  be  used  as  the  pollen  parent  are  removed  from 
the  florets.  Experience  has  shown  that  it  is  best  to  use  only 
anthers  which  are  ready  to  dehisce  and  which  open  after  being 
held  in  the  hand  or  soon  after  being  placed  in  a  watch  glass  in  the 
sun.  A  single  ripe  anther  is  introduced  into  each  floret. 

Where  greenhouse  facilities  are  available,  crosses  may  ad- 
vantageously be  made  in  the  winter  or  early  spring  months. 
This  method  is  used  extensively  by  the  Plant  Breeding  depart- 
ment of  Cornell  University.  When  all  conditions  are  favorable, 
between  50  and  100  per  cent,  of  crossed  seeds  may  be  obtained. 

Barley  and  oats  are  handled  in  nearly  the  same  manner  as 
wheat.  With  barley  it  is  often  necessary  to  emasculate  before 
the  spikes  have  entirely  protruded  from  the  leaf  sheath.  The 
work  is  somewhat  more  difficult,  as  the  flowering  parts  are  much 
more  tender  than  in  wheat.  For  this  reason  forceps  and  shears 
with  very  fine  points  and  thin  blades  are  needed.  Apparently 
under  certain  environmental  conditions  (Arlington  Farm,  Va., 
Norton,  1902)  and  likewise  at  University  Farm,  Minn.,  oat 
flowers  nearly  all  bloom  in  the  late  afternoon.  Artificial 
pollination  under  these  conditions  is  more  easily  performed  in 
the  afternoon  from  one  o'clock  until  mature  pollen  is  no  longer 
easily  collected. 

Among  the  difficulties  of  artificial  crossing  in  the  field  are  un- 
favorable weather  conditions.  Too  much  rain  or  long-continued 
rains  prevent  work.  Jellneck  (1918)  compared  two  methods  of 
crossing  wheats:  (1)  emasculation  and  pollination  by  placing  a 
ripe  anther  in  the  floret;  (2)  emasculating  spikes  as  usual  and 
tying  these  with  spikes  of  similar  maturity  belonging  to  the 
pollen  parent  and  covering  with  a  paper  bag.  In  1916  method 
(2)  gave  twice  as  great  setting  of  seed  as  method  (1).  In  1917 
conditions  were  very  unfavorable  and  no  seed  was  produced  by 


72 


BREEDING  CROP  PLANTS 


method  (1),  while  method  (2)  gave  seeds  in  24  out  of  47  spikes. 
On  these  24  spikes  50  per  cent,  of  florets  produced  seeds. 

Crossing  Large -flowered  Legumes. — Oliver  (1910)  of  the 
United  States  Department  of  Agriculture,  has  made  excellent 
contributions  to  the  technic  of  crossing.  He  emphasizes  the 
fact  that  in  a  cross  between  self -fertilized  varieties,  only  a  few 
seeds  are  needed  in  FI.  The  large-flowered  legumes,  such  as 
Lathyrus,  Phaseolus,  Pisum,  Stizolobium,  and  Vigna,  should  be 
emasculated  in  the  bud  stage.  The  following  account  of  crossing 
Vigna,  the  cowpea,  is  taken  from  Oliver. 

"In  the  evening  it  is  found  that  the  buds  which  will  expand  the  next 
morning  are  quite  large  and  easily  manipulated  in  emasculating  (A). 


FIG.  15. — Flowers  and  young  pods  of  the  cowpea  (twice  natural  size).      (Copied 

from  photograph  by  Oliver.) 

A.  Flower  bud  showing  condition  on  the  evening  of  the  day  previous  to  opening  of  flower; 
B,  flower  in  the  bud  stage  showing  how  the  floral  envelope  is  opened  to  gain  access  to 
stamens  for  emasculation;  C,  flower  with  stamens  removed  showing  the  large  stigrr.a  to 
the  left;  D.  emasculated  flower  the  next  morning  after  pollination;  E,  the  young  pod  the 
second  morning  after  pollination;  F,  the  same  pod  forty-eight  hours  after  the  pollination  of 
the  flower.  (After  Oliver.) 


CONTROLLING  POLLINATION  73 

Hold  the  bud  between  the  thumb  and  forefinger  with  the  keeled  side 
uppermost  (B) ;  then  run  a  needle  along  the  ridge  where  the  two  edges 
of  the  standard  unite.  Bring  down  one  side  of  the  standard,  securing 
it  in  position  with  the  thumb ;  then  do  the  same  with  one  of  the  wings, 
which  will  leave  the  keel  exposed.  This  must  be  slit  on  the  exposed  side 
about  %  in.  below  the  bend  in  the  keel  and  continuing  along  until  about 
He  hi.  from  the  stigma,  which  can  be  seen  through  the  tissue  of  the  keel. 
Bring  down  the  section  of  the  keel  and  secure  it  under  the  end  of  the 
thumb.  This  will  expose  the  immature  stamens,  10  in  number.  With  a 
fine-pointed  pair  of  forceps  seize  the  filaments  of  the  stamens  and  pull 
them  out,  counting  them  as  they  are  removed  to  make  certain  that  none 
are  left  (C).  Allow  the  disturbed  parts  of  keel,  wings,  and  standard  to 
assume  their  original  positions  as  far  as  possible.  Next  detach  a  leaflet 
from  the  plant,  fold  it  once,  place  it  over  the  emasculated  flower  bud, 
and  secure  it  in  position  with  a  pin  or  toothpick." 

This  prevents  drying  out.     Flowers  so  treated  and  pollinated 
the  next  morning  gave  a  large  percentage  of  successful  crosses. 


FIG.  16. — At  right,  A,  scissors  useful  in  removing  small  organs;  B,  self-closing 
forceps;  C,  forceps  commonly  used  in  emasculation  with  pin  attached  to  the 
handle;  D,  scissors  for  severing  large  organs.  At  left,  devices  used  in  depollina- 
tion  of  flowers;  A  and  B,  chip  or  water  bulbs;  C,  water  bulb  with  valve  at  bottom 
provided  with  celluloid  ejector;  D,  old  rubber  bulb  with  glass  tube  inserted; 
E,  "putty  bulb"  with  attachment  to  give  a  small  jet  of  water.  (After  Babcock 
and  Clausen,  1918.  After  Oliver,  1910.) 

Depollination  with  Water. — Oliver  first  used  a  garden  hose  in 
depollinating  Grand  Rapids  lettuce.  By  cutting  down  the  size 
of  the  opening  with  a  smaller  piece  of  rubber  tubing  a  small  jet 
of  water  was  secured.  After  training  this  jet  for  a  few  seconds  on 


74  BREEDING  CROP  PLANTS 

flowers  which  had  just  opened,  no  pollen  remained.  Small 
pieces  of  blotting  paper  were  used  to  remove  excess  moisture  and 
then  pollen  was  applied.  Fifteen  flowers  of  lettuce  were  first 
crossed  by  this  means  and  some  seed  was  produced  in  each 
flower.  The  lettuce  flowers  and  those  of  other  closely  related 
Composite  close  soon  after  pollination. 

Certain  small  rubber-bulb  syringes  have  been  found  satis- 
factory for  field  work.  These  are  used  to  depollinate  the  flowers 
with  water.  .For  a  complete  description  of  artificial  cross- 
pollination  of  alfalfa  flowers  the  reader  is  referred  to  Oliver.  In 
the  flower  to  be  used  as  the  female,  the  anthers  have  already 
dehisced  but  can  not  perform  the  act  of  fertilization  until  the 
flower  is  1  ripped.  To  trip  the  flower  and  secure  as  small  a  per- 
centage of  pollination  as  possible  is  the  aim.  The  technic  of 
tripping  and  depollinating  as  well  as  the  technic  of  crossing  is  a 
matter  of  practice.  Oliver  records  that  more  than  two-thirds  of 
the  alfalfa  pollinations  were  successful  by  this  method. 

Summary  of  Technic  of  Crossing. — Some  important  features 
of  the  technic  of  crossing  may  be  summarized. 

1.  Make  a  careful  study  of  the  structure  of  the  flower  before 
commencing  operations.     This  may  be  done  with  the  aid  of  a 
dissecting  microscope. 

2.  Determine  which  flowers  produce  the  larger,  healthier  seeds 
and  which  set  seeds  the  more  freely. 

3.  Learn  the  normal  method  of  blooming  of  the  flower,  the 
period  of  receptivity  of  the  pistil,  and  the  length  of  time  the 
pollen  grains  are  capable  of  functioning. 

4.  Procure  the  necessary  tools  and  see  that  these  are  of  an 
efficient  kind  for  the  work  to  be  undertaken. 

5.  Be  careful  not  to  injure  the  flowering  parts  any  more 
than  is  necessary.     Do  not  remove  the  surrounding  flower  parts, 
i.e.,  petals  in  flowering  plants,  glumes  of  grasses,  etc.  unless 
necessary. 

6.  A  few  crosses  well  made  are  of  much  greater  value  than 
many  pollinations  carelessly  executed. 


CHAPTER  VI 
CLASSIFICATION  AND  INHERITANCE  IN  WHEAT 

Studies  of  genetics  have  led  to  the  adoption  of  a  particular 
meaning  which  is  understood  when  we  speak  of  an  inherited 
character.  It  is  the  final  result  of  the  interaction  of  many 
inherited  factors  plus  the  environment.  The  factors  are  the 
inheritance  and  the  ultimate  character  is  the  manner  of  reaction 
under  the  special  growing  conditions  to  which  the  organism  is 
subjected.  What  is  inherited  is  the  ability  to  react  in  a  particu- 
lar manner  in  a  given  place  and  not  the  character  itself. 

Genetic  Classification. — Classification  of  cultivated  varieties  of 
crops  is  made  in  much  the  same  manner  as  the  botanical  classifica- 
tion of  wild  species.  With  crops,  there  is  as  a  rule  considerable 
experimental  evidence  of  genetic  relationship.  The  ultimate 
aim  of  crop  classification  should  be  genetic  in  order  that  it  may  be 
of  greatest  value.  Closeness  of  relationship  as  determined  by 
the  ease  of  crossing  and  the  degree  of  sterility  is  frequently  made 
the  basis  of  species  groups  in  some  crops.  In  other  crops  no 
sterility  is  obtained  in  so-called  species  crosses.  Only  relatively 
stable  characters  which  are  not  easily  modified  under  different 
environmental  conditions  are  considered  of  major  classification 
value. 

After  placing  cultivated  crops  in  groups  which  are  roughly 
analogous  to  botanical  species,  the  next  step  is  more  clearly 
to  separate  different  categories  of  a  lower  order  of  classification. 
These  are  the  varieties.  Varieties  are  not  necessarily  genetic 
entities  but  may  be  groups  of  similar  forms  which  resemble  each 
other  more  than  individuals  belonging  to  another  variety.  All 
members  of  a  variety  are  similar  to  each  other  in  major  botanical 
characters. 

Such  a  variety  classification  is  of  utmost  importance.  In  the 
past  the  variety  studies  made  in  the  United  States  by  the  different 
experiment  stations  or  the  federal  Department  of  Agriculture 
have  not  always  been  comparable,  as  the  same  name  has  been 
used  to  refer  to  widely  different  varieties.  More  dependable  results 
can  only  be  obtained  by  the  adoption  of  uniform  variety  names. 

75 


76  BREEDING  CROP  PLANTS 

Classifications  of  some  crops  have  recently  been  made  and  in  the 
next  few  years  these  will  be  improved  further.  The  general 
adoption  of  some  standard  variety  classification  is  a  necessity  if 
work  of  different  investigators  in  crops  is  to  be  correlated. 

The  central  aim  in  crop  improvement  work  is  to  find  or  produce 
improved  forms  which  when  grown  by  farmers  will  excel  in 
quality,  productivity,  or  ease  of  handling.  It  is  a  decided 
advantage  if  the  improved  form  can  be  distinguished  from  the 
varieties  commonly  grown  in  the  locality  by  some  botanical  or 
morphological  character  difference.  Kanred  (Jardine,  1917) 
wheat  is  an  example  of  a  new  variety  with  such  a  character. 
This  variety,  which  was  developed  at  the  Kansas  station,  belongs 
to  the  Crimean  group  of  winter  wheats.  It  gives  larger  yields 
on  the  average  than  Turkey  or  Kharkov  selections  and  excels  in 
resistance  to  black  stem  rust,  Puecinia  graminis  tritici,  and 
leaf  rust,  Puecinia  triticini.  Its  beak,  i.e.,  the  extension  of  the 
outer  glume  in  the  form  of  an  awn  point,  is  longer  than  in  the 
common  forms  of  Crimean  winter  wheat  grown  in  Kansas. 
Marquis  wheat,  which  is  so  widely  grown  as  a  spring  wheat  in  the 
Northwest  and  Canada,  differs  in  seed  shape  from  other  Fife 
wheats  commonly  grown  in  these  sections.  Forms  belonging;  to 
the  same  variety  may  frequently  exhibit  differences  in  productiv- 
ity and  this  may  be  the  sole  distinguishing  character  difference. 
Forms  constantly  differing  from  each  other  in  one  or  more  genetic 
factor  differences  which  may  be  expressed  as  yield,  quality,  or 
disease  resistance,  or  a  minor  botanical  character  and  yet  which 
belong  to  the  same  variety  group,  may  be  called  strains.  This 
is  the  lowest  order  of  classification  which  can  be  adopted  for  seeded 
crops.  With  a  self-fertilized  crop  the  strain  may  also  be  a  pure- 
line  in  the  original  sense  as  used  by  Johannsen.  With  cross- 
fertilized  crops  the  strain  may  be  relatively  pure  for  some  particu- 
lar character  and  may  be  heterozygous  for  other  characters. 

Inheritance  studies  of  many  of  our  farm  crops  have  been  made. 
As  crossing  is  the  only  means  of  inducing  variation  that  can  be 
carried  out  with  success  by  the  plant  breeder,  it  becomes  neces- 
sary to  know  how  individual  characters  are  inherited.  It  is  true 
that  yield  is  not  a  simple  Mendelian  character  but  is  dependent 
on  many  inherited  factors  and  their  manner  of  reaction  to  the 
environment.  At  present,  knowledge  of  inheritance  may  be 
used  only  as  a  guide  in  working  with  these  characters.  As  a  rule, 
the  parental  forms  differ  in  botanical  characters  as  well  as  in 


CLASSIFICATION  AND  INHERITANCE  IN  WHEAT        77 


yielding  ability.  A  knowledge  of  the  mode  of  inheritance  of 
each  of  these  characters  is  essential  to  the  rapid  purification  of  a 
cross. 

It  is  not  desirable  in  a  text  on  plant  breeding  to  outline  variety 
classifications  in  very  great  detail.  As  a  rule  the  crops  student 
will  be  familiar  with  such  classifications  before  studying  crop 
improvement.  It  seems  sufficient  to  indicate  genetic  relation- 
ship and  to  point  out  the  characters  which  have  been  used. 

Wheat  Species  Groups. — From  the  middle  of  the  last  century 
until  the  present  time  numerous  crosses  between  wheat  varieties 
and  also  between  species  groups  have  been  made.  Extensive 
crossing  studies  have  led  Tschermak  (1914  a,b)  to  conclude  that 
the  genetic  relationships  in  wheat  areas  represented  in  Table  XV. 

TABLE  XV. — WHEAT  SPECIES  GROUPS 


Group  composition 

Einkorn  group 

Emmer  group                 Spelt  group 

Stem  species 
Spelt  wheats 

T.  aegilopoides 

T.  dicoccoides 

T.  spelta 
wild     form    un- 
known 

Cultivated  forms 
Covered  seed 

T.  monococcum 

T.  dicoccum 

T.  spelta 

Cultivated  forms 
Naked  seed 

Unknown 

T.  turgidum 
T.  polonicum 
T.  durum 

T.  vulgare 
T.  compactum 

Crosses  reported  by  Tschermak  between  the  einkorn  and  spelt- 
groups  so  far  have  proved  wholly  sterile,  while  the  einkorn s 
emmer  crosses  have  proved  only  slightly  fertile.  Similar  result, 
have  been  obtained  by  other  investigators.  The  crosses  between 
the  covered  emmer  types  and  the  naked  and  covered  spelt  forms 
or  between  covered  and  naked  forms  of  the  emmer  group  were 
partially  fertile.  Somewhat  greater  fertility  was  found  in 
crosses  between  T.  polonicum  and  the  naked  wheats  of  the  spelt 
group,  also  between  naked  forms  of  the  emmer  group  and  the 
covered  form  of  the  spelt  group.  Some  of  the  latter  crosses 
seemed  wholly  fertile.  Crosses  belween  naked  wheats  proved 
wholly  fertile. 

Vilmorin  (1880,  1883)  concluded  that  spelt  and  common  wheats 
belong  to  one  group  and  durum  and  turgidum  to  another,  for 


78 


BREEDING  CROP  PLANTS 


crosses  between  any  form  in  the  first  group,  with  any  form  in  the 
second  group  gave  all  cultivated  forms  of  the  spelt  and  emmer 
groups  in  later  generations.  Tschermak  (1913)  obtained  similar 
results  only  from  crossing  solid  and  hollow  stemmed  varieties  of 
the  respective  groups  and  only  obtained  polonicum  forms  when 
using  polonicum  as  one  of  the  parents. 


FIG.  17. — Wild  wheat  from  Palestine  and  the  New  Hybrid.  Here  is  shown  a 
spikelet  of  the  true  wild  wheat  and  one  of  the  hybrid  forms.  (After  Love  and 
Craig,  1919.) 


T.  dicoccoides  was  reported  as  being  found  wild  as  early  as 
1885.  Aaronsohn  (1910)  found  many  wild  forms  of  T.  dicoccoides 
in  Palestine.  Love  and  Craig  (19196)  have  produced  T.  dicoc- 
coides synthetically  by  crossing  durum  and  common  varieties, 
which  indicates  rather  close  genetic  relationships  between  these 
forms.  There  seems  no  very  good  reason  to  the  writers  for 


CLASSIFICATION  AND  INHERITANCE  IN  WHEAT        79 

concluding  that  the  cultivated  einmer  and  spelt  groups  arose 
from  different  wild  stem  species.  It  is  also  essential  to  point  out 
that  all  crosses  between  the  cultivated  naked  emmer  wheats  with 
naked  wheats  belonging  to  the  spelt  group  are  not  entirely  fertile. 
Indications  of  partial  sterility  are  generally  apparent  if  the  results 
are  carefully  analyzed  (Kezer  and  Boyack,  1918)  (Freeman,  1919) 
(Hayes  and  others,  1920). 

Polonicum  Crossed  with  Other  Species. — Crosses  between 
polonicum  and  other  forms  have  been  studied.  Tschermak 
(1913),  in  a  cross  between  polonicum  and  vulgare,  explained  the 
results  by  two  main  factor  differences.  The  FI  was  of  inter- 
mediate glume  length  and  in  Fz  polonicum,  durum,  and  vulgare 
forms  were  obtained  as  well  as  intermediates.  Pure  polonicum 
was  considered  to  contain  two  dominant  factors  in  the  homo- 
zygous  condition;  durum,  one  dominant  factor  pair  in  the  homo- 
zygous  condition;  and  the  pure  vulgare  forms,  both  factors  in 
the  recessive  condition. 

Polonicum  (Backhouse,  1918)  crossed  with  durum  or  turgi- 
dum  gave  intermediate  glume  length  in  FI  and  segregation  in 
F2  in  a  ratio  of  3  longs  and  intermediates  to  1  short.  Biffen 
(1916)  and  Backhouse  in  separate  studies  considered  the  factor 
for  polonicum  glume  to  inhibit  chaff  pubescence  and  color.  In  a 
cross  between  durum  (Kubanka)  with  a  polonicum  variety,  the  F% 
segregated  for  glume  length  and  hairy  chaff.  The  short-glumed 
plants  were  in  a  ratio  of  3  hairy  to  1  smooth,  while  the  long- 
glumed  plants  were  difficult  to  classify  for  condition  of  chaff. 
Crosses  of  different  long-glumed  plants  with  other  wheats  showed 
that  a  part  of  these  long-glumed  wheats  contained  a  genetic 
factor  for  hairy  chaff.  Results  were  explained  on  the  hypothesis 
that  the  factor  for  long  glume  partially  inhibited  development 
of  hairy  chaff.  Similar  results  were  obtained  by  Biffen  (1916), 
for  inhibition  of  glume  color  by  the  polonicum  factor  for  glume 
length. 

Some  Linkage  Results  in  Wheat  Crosses. — In  crosses  be- 
tween the  different  species  some  evidences  of  linkage  have 
been  observed.  In  turgidum-vulgare  crosses,  Biffen  (1905)  ob- 
tained complete  linkage  of  gray  color  of  glumes  with  hairy  chaff. 
Engledow  (1914)  crossed  a  black-glumed  wheat  obtained  from  a 
turgidum-fife  cross  with  a  rough-chaffed,  white-glumed  variety, 
Essex  Rough  Chaff.  The  ratio  obtained  in  Fz  was  explained  on 
the  basis  of  repulsion  between  the  factors  for  black  glume  color 


80  BREEDING  CROP  PLANTS 

and  those  for  hairy  chaff  on  the  1:3:3:1  series.  Kezer  and 
Boyack  (1918)  obtained  complete  linkage  of  black  and  hairy 
chaff  in  a  cross  of  black  winter  emmer  with  a  smooth,  white- 
chaffed  winter  wheat  (T.  vulgar  e).  Freeman  (1917)  obtained 


FIG.  18. — Upper  group  from  left  to  right:  Face  and  side  views  respectively 
of  lumillo  durum  (C.I.,  1736),  F\  lumillo  X  Marquis,  and  Marquis.  The  F\ 
spikes  are  intermediate  in  density,  have  tipped  awns  and  the  outer  glumes  are 
keeled  although  not  so  strongly  as  lumillo.  Lower  group,  left  to  right,  face  and 
side  views  respectively  of  Emmer,  Minn.,  1165,  F\  Emmer  X  Marquis,  and 
Marquis.  The  F\  approaches  the  Emmer  in  some  spike  characters  and  has 
tipped  awns. 

some  correlation  between  a  high  ratio  of  width  to  thickness  of 
spike  and  hardness  of  grain  in  crosses  between  T.  durum  and  T. 
vulgare.  He  considers,  however,  that  numerous  factors  are 
necessary  for  the  development  of  these  characters. 


CLASSIFICATION  AND  INHERITANCE  IN  WHEAT        81 

Spike  Density.  —  Compactness  of  spike,  color  of  seed  and 
chaff,  texture  of  seed,  and  presence  or  absence  of  awns  are  fre^ 
quently  used  in  wheat  variety  classification. 

Nilsson-Ehle  (19116),  in  crosses  between  compact  and  square- 
head (mid-dense)  wheats,  obtained  compact  forms  in  FI  and 
segregation  into  compact,  mid-dense  and  lax  in  Fz.  He  explained 
the  results  by  supposing  the  main  factor  differences  to  be  as 
follows; 

Swedish  Binkel  (compact)  CCLiLiL2Z/2 
Squarehead  c 


The  C  factor  was  considered  to  inhibit  the  expression  of  the 
lengthening  factors  L\  and  L2,  and  also  to  produce  spikes  with 
short  internodes.  While  these  factors  gave  a  satisfactory 
explanation  of  his  crosses  Mayer  Gmelin  (1917)  showed  that  they 
did  not  explain  the  production  of  compact  spiked  forms  which  he 
obtained  from  crosses  of  spelt  (lax)  and  Essex  Velvet  Chaff, 
which  is  mid-dense.  In  F%  generations  grown  from  individual 
plants  of  a  cross  between  white  spring  emmer  and  Marquis, 
studied  at  the  Minnesota  Experiment  Station,  a  very  common 
sort  of  segregation  was  from  lax,  keeled,  speltlike  wheats  to 
compact,  keelless,  naked  wheats.  This  might  indicate  that 
spelt  wheats  contain  a  compact  factor  which  is  prevented  from 
expression  by  some  other  genetic  factor. 

Crosses  between  T.  compactum  and  T.  vulgare  by  Spillman 
(1909)  and  Gaines  (1917)  have  shown  one  main  factor  difference 
for  compactness.  Parker  (1914)  made  careful  measurements  of 
internode  length  in  crosses  of  wheats  belonging  to  T.  compactum 
with  those  of  T.  vulgare.  He  was  able  to  demonstrate  segregation 
but  found  the  condition  very  complex.  Results  of  this  nature 
have  been  satisfactorily  explained  by  the  multiple  factor  hy- 
pothesis. The  number  of  factors  involved  cannot  accurately 
be  determined.  Nilsson-Ehle,  likewise,  states  that  besides  the 
main  factor  differences  there  are  other  minor  factors  which 
influence  spike  density  and  account  for  a  wide  range  of  homozy- 
gous  forms. 

Seed  Characters.  —  Color  of  seed,  which  results  from  a  brown- 
ish, red  pigment  in  one  of  the  bran  layers  (remains  of  nucellus) 
has  been  quite  consistently  used  in  variety  classification.  This 
is  a  plant  character  and  not,  therefore,  immediately  affected  by 
pollination.  Red  is  dominant  over  white  and  in  the  second 

6 


82  BREEDING  CROP  PLANTS 

generation  a  ratio  of  3  red-seeded  plants  to  1  white-seeded  plant 
is  often  obtained.  Nilsson-Ehle  (19116)  was  the  first  writer 
who  reported  crosses  which  in  F2  gave  15  to  1  or  63  to  1  ratios  of 
red-  and  white-seeded  plants.  The  Howards  (1912),  in  India, 
have  obtained  63:1  ratios  in  crosses  of  American  Club  with  pure 
lines  of  Indian  wheats,  and  Gaines  (1917)  in  Washington,  has 
obtained  similar  results  from  a  cross  between  Bluestem  (red  seed) 
and  Brown's  Glory  (white  club  wheat).  Nilsson-Ehle  obtained 
a  ratio  of  15  red-seeded  plants  to  1  white-seeded  plant  from  a 
cross  of  two  red-seeded  varieties.  The  inheritance  of  this  seed 
color  has  been  explained  by  one  or  more  Mendelian  factors, 
each  when  present  giving  red  and  when  absent  white.  The 
factors  are  separately  inherited,  each  when  homozygous  produc- 
ing somewhat  darker  color  than  when  heterozygous.  They  are 
also  cumulative,  two  factors  giving  a  darker  color  on  the  average 
than  one  of  these  factors  alone.  It  is  impossible,  by  inspection, 
to  determine  how  many  factors  are  responsible  for  a  particular 
varietal  seed  color. 

Texture  of  seed  has  also  been  used  in  varietal  classification  and 
is  a  character  which  determines  to  some  extent  the  market  class 
in  which  the  variety  will  be  placed.  Biff  en  (1916)  found  im- 
mediate effect  of  pollination  in  a  cross  of  Rivet,  a  hard-seeded 
turgidum  with  pollen  from  a  soft  Polish  variety.  The  FI  genera- 
tion plants  produced  hard  seed  and  the  F%  segregated  into 
hard-  and  soft-seeded  plants  in  a  ratio  of  3:1.  The  Howards 
(1915)  obtained  an  intermediate  condition  inFi  plants  and  a  1 : 2 : 1 
ratio  in  F2  in  crosses  between  hard-  and  soft-seeded  strains. 
Freeman  (1918)  crossed  hard-seeded  durums  with  T.  vulgare, 
variety  Sonora,  a  soft-seeded  wheat.  The  FI  plants  produced 
hard,  intermediate,  and  soft  seeds.  The  hard  seeds  of  the  FI 
tended  to  give  more  hard-seeded  plants  in  F2,  and  the  soft-seeded 
tended  to  give  more  soft-seeded  plants.  Freeman  carried  the 
study  through  ^4.  He  explained  his  results  on  the  basis  of  two 
factors  for  starchiness,  each  inherited  independently.  He 
supposed  each  to  produce  half  as  much  soft  starch  when  hetero- 
zygous as  when  homozygous.  As  the  endosperm  is  the  result  of 
the  fusion  of  two  polar  nuclei  with  one  of  the  male  generative 

FIG.  19. — Representative  spikes  of  F3  families  of  the  cross  between  Durum 
and  Marquis.  Upper  4  groups,  Fa  families  which  were  classified  as  durums. 
Note  that  they  represent  all  types  of  spike  density.  Lower  left,  spikes  of  an 
awnless  F3  Emmer  family.  Lower  right,  four  spikes  of  an  F3  plant  which  re- 
sembled common  wheat  in  spike  shape  and  which  proved  rust  resistant. 


CLASSIFICATION  AND  INHERITANCE  IN  'WHE'AV 


FIG.  19. 


84  BREEDING  CROP  PLANTS 

nuclei,  there  may  be  a  range  of  from  0  to  6  factors  for  starchiness 
of  the  endosperm.  This  assumption  was  shown  to  explain  results 
quite  satisfactorily.  The  above  starchiness  is  believed  by  Free- 
man to  be  quite  different  from  the  well-known  "yellow  berry"  of 
wheat.  Numerous  workers  have  shown  that  varieties  and  strains 
differ  widely  in  the  amount  of  "yellow  berry"  when  grown  under 
the  same  environmental  conditions.  Texture  of  seed  is,  however, 
a  character  which  is  quite  easily  modified  by  unfavorable  environ- 
mental conditions. 

Chaff  Characters. — There  are  a  number  of  different  intensities 
of  the  chaff  color.  In  some  cases  a  deep  brownish  red  color  is 
present,  in  other  cases  a  light  brownish-red,  and  frequently  the 
outer  glumes  have  dark  brownish  red  striations  on  a  slightly 
colored  or  colorless  background.  Biff  en  (1905)  studied  crosses 
between  so-called  red  and  colorless  and  obtained  red  or  reddish 
color  in  FI  and  a  3:1  segregation  of  colored  to  colorless  in  F2. 
Kezer  and  Boyack  (1918),  in  winter  wheat  crosses  in  which  the 
parents  differed  in  chaff  color,  obtained  intermediate  color  in  FI 
and  segregation  in  a  3:1  ratio  in  F2.  Simple  ratios  in  varietal 
crosses  have  been  reported  by  others  for  this  color  character. 
As  there  are  different  intensities  which  are  quite  uniform  in  in- 
heritance it  seems  reasonable  to  conclude  that  there  are  different 
factors  in  different  varieties  for  brownish-red  color.  In  a  durum- 
vulgare  cross,  Love  and  Craig  (1918a)  obtained  in  F2  an  indica- 
tion of  a  15: 1  ratio  for  brownish  red  and  colorless  chaff. 

Besides  the  chaff  colors  there  are  awn  colors.  The  Howards 
(1915),  in  India,  obtained  a  ratio  in  F2  of  3.45  black-awned  to  1 
colorless  in  a  cross  between  Indian  wheats. 

Hairy  chaff  is  a  varietal  character  of  considerable'  classification 
value.  The  Howards  have  made  extensive  studies  of  this 
character.  Under  linkage  relations  a  number  of  cases  were 
given  in  which  hairy  chaff  was  correlated  with  glume  color. 
Henkemeyer  (1915)  reports  different  crosses,  one  in  which  hairy 
chaff  is  correlated  with  white  chaff  and  another  in  which  these 
characters  are  independently  inherited.  This  leads  one  to  sus- 
pect that  there  are  two  kinds,  either  of  hairy  chaff  or  of  chaff  color. 
The  Howards  have  been  able  to  demonstrate  two  kinds  of  hairs 
on  the  glumes  of  Rivet  wheat.  Two  Indian  varieties  were  like- 
wise studied.  Each  produced  hairy  chaff,  but  differed  in  the 
sort  of  hairs  produced.  In  crosses  between  these  varieties, 
ratios  of  15  pubescent  to  1  smooth  were  obtained  in  F2. 


CLASSIFICATION  AND  INHERITANCE  IN  WHEAT        85 

Presence  or  Absence  of  Beards. — Wheats  have  been  classified 
as  bearded  and  awnless  but  this  is  not  genetically  correct.  The 
awn  is  an  extension  of  the  flowering  glume.  The  common  wheats, 
like  Marquis  and  Bluestem,  are  not  truly  awnless  for  there  is  a 
short  extension  of  the  awn  particularly  in  the  spikelets  at  the  top 
of  the  spike.  Three  to  one  ratios  have  generally  been  obtained  in 
crosses  between  bearded  and  so-called  awnless  (tip-awned) 
wheats.  The  Howards  (1915)  have  carefully  worked  out  the 
inheritance  of  these  characters.  They  have  explained  results  by 
supposing  two  factors,  A  and  B,  to  be  present  in  a  homozygous 
condition  in  bearded  wheats.  They  have  found  two  kinds  of  very 
short-awned  wheats,  one  like  the  tip-awned  Marquis  or  Bluestem, 
and  the  other  with  somewhat  longer  tip  awns.  Each  of  these 
varieties  was  found  to  contain  one  of  the  factors  A  or  B  in  a 
homozygous  condition.  In  crossing  a  tip-awned  wheat  like 
Marquis  with  bearded  varieties,  theFi  generation,  as  a  rule,  shows 
an  extension  of  the  tip  awns  and  it  is  frequently  possible  to  separate 
these  FI  plants  from  the  tip-awned  parent.  In  crossing  bearded 
with  true  beardless,  the  PI  is  apparently  beardless  and  there  is  a 
range  in  F2  from  completely  bearded  to  awnless.  Fully  bearded 
plants  breed  true  for  this  character. 

Inheritance  of  Disease  Resistance.— Biffen  (1907a,  1912,  1917) 
has  found  that  the  inheritance  of  host  reaction  to  stripe  rust, 
Puccinia  glumarum,  is  a  simple  Mendelian  character.  Suscep- 
tibility is  dominant  over  resistance  and  in  F2,  ratios  of  3  suscep- 
tible to  1  resistant  are  obtained.  Nilsson-Ehle  (191  Ib)  in  a 
similar  study  found  the  FI  generation  resembled  the  susceptible 
parent  in  some  cases,  the  resistant  in  others,  and  was  inter- 
mediate in  still  others.  Complex  segregation  for  resistant 
versus  susceptible  forms  was  obtained  in  later  generations. 
Results  were  explained  on  the  multiple  factor  basis. 

Studies  by  Stakman  and  others  (1919)  have  shown  the  prob- 
able reason  for  conflicting  reports  regarding  inheritance  of  resist- 
ance to  black  stem  rust  of  wheat,  Puccinia  graminis  tritici.  They 
have  demonstrated  the  fact  that  there  are  a  number  of  biological 
or  racial  forms  of  rust  roughly  analogous  to  pure  lines.  These 
forms  can  only  be  differentiated  surely  by  their  specific  reaction 
to  pure-line  wheat  varieties.  Studies  of  their  constancy  indicate 
that  they  are  not  easily  modified,  i.e.,  that  the  parasitic  reaction 
of  each  form  is  constant.  At  the  Minnesota  Station  (Hayes  and 
others,  1920)  studies  of  inheritance  of  resistance  were  made  in 


86 


BREEDING  CROP  PLANTS 


crosses  between  resistant  emmers  and  durunis  with  susceptible 
Marquis.  A  single  rust  form  was  used  in  making  the  artificial 
epidemic.  The  durum-Marquis  crosses  were  as  susceptible  as 
Marquis  in  FI.  Using  white  spring  emmer  as  the  resistant  parent, 
the  FI  was  resistant,  though  not  so  resistant  as  the  emmer  parent. 
Segregation  for  resistance  and  botanical  characters  was  studied 
in  later  generations.  Some  linkage  in  transmission  was  apparent, 
for  while  it  was  quite  easy  to  obtain  resistant  emmer  or  durum 


FIG.  20. — Resistance  of  parents  and  crosses  to  a  strain  of  stem  rust.  From 
left  to  right:  Culms  of  resistant  Durum  wheat;  FI  of  Durum  X  Marquis, 
susceptible;  Marquis,  susceptible;  F\  of  Emmer,  Minn.  1165  X  Marquis,  as 
resistant  as  the  Durum  varieties;  Emmer,  Minn.  1165,  a  very  resistant  variety. 

plants  it  was  much  more  difficult  to  obtain  resistant  common 
wheats.  In  an  examination  of  more  than  20,000  F$  plants,  a  few 
with  vulgar e  spike  characters  and  resistance  were  obtained. 
Resistant  plants  resembling  emmer,  durum,  and  common 
wheats  were  also  proved  resistant  by  greenhouse  inoculation 
studies. 

Gaines  (1918,  1920)  has  studied  the  inheritance  of  resistance 
of  wheats  to  bunt  (Tilletia  tritici).  It  is  estimated  that  this 


CLASSIFICATION  AND  INHERITANCE  IN  WHEAT        87 

disease  causes  an  annual  decrease  of  15  per  cent,  in  the  yield  of 
winter  wheat  in  the  States  of  Washington,  Oregon,  and  Idaho. 

The  method  used  in  studying  bunt  resistance  was  to  blacken 
the  seed  with  smut  spores  just  before  planting  and  then  sow  the 
strains  and  crosses  in  rows  in  such  a  way  that  each  plant  could 
be  individually  examined. 

Studies  of  inheritance  were  made  in  crosses  of  Turkey  X  Flor- 
ence and  Turkey  X  Hybrid  128.  Hybrid  128  is  a  prolific, 
winter-hardy,  stiff-strawed  wheat  of  much  commercial  value 
but  it  is  very  susceptible  to  bunt.  Turkey  does  not  yield  as  well 
as  Hybrid  128,  has  weak  straw,  and  shatters  considerably.  It  is 
highly  resistant  to  bunt.  Florence  is  an  Australian  spring  wheat 
which  is  highly  resistant  to  bunt. 

The  results  presented  by  Gaines  show  very  clearly  that  resist- 
ance to  bunt  is  an  inherited  character.  However,  several  factors 
are  necessary  to  explain  the  sort  of  segregation  obtained.  The  Fz 
of  the  cross  between  Florence  and  Turkey  showed  transgressive 
segregation.  In  Fs,  171  families  were  grown  from  individual  F2 
plants.  Of  these,  72  were  immune  while  50  families  produced  over 
80  per  cent,  of  bunt.  The  Turkey  and  Florence  parents  under 
the  same  conditions  produced  an  average  of  4.6  per  cent,  of 
infected  plants.  This  shows  that  the  factors  for  resistance  in  the 
Florence  and  Turkey  varieties  are  not  identical. 

In  the  cross  between  Turkey  and  Hybrid  128  no  segregates 
were  obtained  with  a  higher  degree  of  resistance  than  the  Turkey 
parent.  It  was  found  possible  to  produce  resistant  strains  of 
any  morphological  type  desired. 

Inheritance  of  Other  Characters.— Nilsson-Ehle  (1911c,  1912) 
has  shown  that  winter-hardiness  is  inherited  in  much  the  same 
manner  as  other  characters.  Segregation  occurs  in  Fz  and  types 
can  be  produced  in  later  generations  which  are  homozygous  for 
different  degrees  of  winter-hardiness.  Crosses  made  (Hayes  and 
Garber,  1919)  in  1902  between  hardy  Odessa  winter  wheat  and 
Turkey  varieties  were  bred  for  several  years  by  continuous  selec- 
tion methods.  Odessa  is  a  late  maturing  variety  and  does  not 
give  a  high  yield  in  Minnesota.  Turkey  is  a  desirable  winter 
wheat  in  many  sections  but  it  lacks  hardiness  under  Minnesota 
conditions.  Two  early  maturing  wheats,  Minhardi  and  Min- 
turki,  have  been  produced  from  the  cross  between  Turkey  and 
Odessa.  These  new  varieties  excel  in  winter-hardiness  and 
yield. 


88  BREEDING  CROP  PLANTS 

The  Howards  (1915)  state  that  standing  power  is  due  to  a 
combination  of  a  strong  root  system  and  stiff  straw  and  report 
segregation  and  recombination  in  a  cross  between  two  varieties, 
each  of  which  contained  one  character  and  lacked  the  other.  It 
was  impossible  to  determine  the  factors  involved.  Spillrnan 
(1909)  made  a  cross  between  a  winter  wheat  with  weak  straw  and 
spring  wheat  with  stiff  straw  and  obtained  in  later  generation  a 
winter  wheat  with  stiff  straw.  Examples  of  inheritance  of  other 
similar  characters  could  be  given.  It  is  reasonable  to  conclude 
that  those  growth  characters  which  determine  the  productive 
capabilities  of  each  variety  are  inherited  in  the  same  manner 
as  botanical  characters.  They  are  due,  generally,  to  the  inter- 
action of  numerous  factors  which  are  dependent  for  their  full 
expression  on  favorable  environmental  conditions. 


CHAPTER  VII 

CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS 
OTHER  THAN  WHEAT 

In  the  cases  of  barley  and  oats  quite  usable  classifications  have 
been  proposed.  The  general  adoption  of  such  classification 
schemes  is  desirable  for  often  great  confusion  results  from  the 
incorrect  use  of  varietal  names.  Classification  schemes  can 
not  be  given  in  detail  in  a  plant  breeding  text.  It  seems  sufficient 
here  to  point  out  the  genetic  relationship  between  wild  and 
cultivated  species  and  to  give  the  major  so-called  species  groups 
for  the  various  crops.  The  more  important  botanical  and 
agronomic  characters  which  are  commonly  used  in  varietal 
classification  have  also  been  mentioned.  As  crossing  must 
frequently  be  resorted  to  as  a  means  of  improving  small  grains, 
the  student  should  have  a  working  knowledge  of  the  known  facts 
of  inheritance  with  respect  to  particular  characters. 

CLASSIFICATION  AND  INHERITANCE  IN  OATS 

A  workable  classification  of  cultivated  American  oat  varie- 
ties and  the  basic  wild  species  has  been  made  by  Etheridge 
(1917).  The  following  outline  of  species  groups  is  taken  from 
his  publication; 

A.  Kernel  loose  within  the  surrounding  hull;  lemma  and  glumes  alike  in 

texture. •  •  •  •  '.Avena  nuda. 

A  A.  Kernel  firmly  clasped  by  the  hull;  lemma  and  glumes   different  in 
texture. 

B.  Upper  grains  persistent  to  their  rachillas Avena  sterilis. 

BB.  Upper  grains  easily  separating  from  their  rachillas. 
C.  Lemma  bearing  as  teeth  or  awn  points. 

D.  Lemma  with  four  teeth  or  awn  points. 

Avena  abyssinica. 
DD.  Lemma  with  two  teeth  or  awn  points. 

E.  Lemma  elongate,   lanceolate,  with  distinct  awn 

points Avena  strigosa. 

EE.  Lemma  short,  abrupt,  blunt,  rather  toothed  than 

awn-pointed . . ; Avena  brevis. 

89 


90  BREEDING  CROP  PLANTS 

CC.  Lemma  without  teeth  or  awn  points. 

D.  Basilar  connections  of  the  grains  articulate 

Avena  fatua. 

DD.  Basilar  connections  of  the  grains  solidified. 
E.  Panicles  roughly  equilateral,  spreading. 

Avena  saliva. 
EE.  Panicles  unilateral,  appressed. 

Avena  saliva  orientalis. 

Crosses  Between  Avena  fatua  and  A.  sativa. — It  is  generally 
accepted  that  fatua  is  the  stem  species  from  which  A .  sativa  and 
A.  sativa  orientalis  originated.  Tschermak  (1914)  has  made 
extensive  crosses  and  obtained  nearly  complete  fertility  in  crosses 
between  fatua  and  sativa  forms.  Surface  (1916)  has  found  a 
number  of  characters  which  in  crosses  between  fatua  and  sativa 
are  associated  with  the  fatua  base— -(1)  heavy  awn  on  lower 
grain,  (2)  awn  on  upper  grain,  (3)  fatua  base  on  upper  grain,  (4) 
pubescence  on  rachilla  of  lower  grain  and  upper  grain,  (5) 
pubescence  on  all  sides  of  the  base  of  lower  grain  and  pubescence 
on  the  upper  grain. 

Origin  of  the  Cultivated  Varieties  of  A.  sterilis.— Norton  (1907) 
pointed  out  that  the  red  oats  grown  in  southern  United  States 
without  doubt  descended  from  A.  sterilis  of  the  Mediterranean 
region.  Trabut  (1914)  gives  quite  convincing  evidence  that  the 
cultivated  oats  of  the  Mediterranean  region  have  been  obtained 
from  a  wild  A.  sterilis,  which  is  still  quite  common.  It  is  of 
interest  to  the  student  of  plant  breeding  that  the  cultivated  oats 
grown  in  the  warmer  regions  of  the  United  States  descended  from 
warm-climate  ancestors.  The  value  of  this  group  of  oats  for  the 
southern  United  States  has  clearly  been  shown  by  Warburton 
(1914). 

Differences  in  Awn  Development. — Varieties  of  oats  differ 
in  the  presence  or  absence  of  awns  and  in  the  degree  of  awn 


FIG.  21. 

1.  Branch  of  oat  panicle. 

2.  Spikelet,   showing  tertiary  floret  just  after  blooming — a,   primary  floret. 

3.  Spikelet,   showing  flower  parts — a,   outer   glume;   b,   flowering   glume;   c, 
palea;  d,  lodicules;  e,  anther;  /,  stigma;  0,  secondary  floret;  h,  awn. 

4.  Outer  parts  removed,  showing  sexual  organs. 

5.  Longitudinal  section  ovary. 

6.  Anther. 

7.  Showing  outer  and  flowering  glume  of  lower  spikelet  removed — a,  lodicules, 
and  sexual  organs. 

Size:  1,  2,  about  n;  3,  about  2n;  4,  5,  6,  greatly  enlarged;  7,  about  2n. 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    91 


FIG.  21. 


92  BREEDING  CROP  PLANTS 

development.  Nilsson-Ehle  (1911a)  first  used  the  hypothesis 
that  the  yellow  gene  inhibited  the  development  of  awns.  This 
hypothesis  was  substantiated  by  careful  experiments.  A  num- 
ber of  crosses  betwen  Avena  fatua,  hairy  awns  on  both  grains, 
with  early  oats  belonging  to  the  Avena  saliva  group  have  been 
studied.  Using  Sixty  Day  with  yellow  grains  as  the  awn- 
less  parent,  Love  and  Craig  (1918c)  observed  the  FI  to  have 
the  lower  grain  often  awned  but  the  upper  grain  awnless.  They 
concluded  that  the  yellow  factor  inhibited  the  complete  develop- 
ment of  awns.  In  a  similar  cross,  Surface  (1916)  obtained  like 
results  in  FI  and  concluded  that  one  main  factor  difference  was 
necessary  to  explain  the  results.  Modifying  factors  were  involved 
which  affected  the  degree  of  development  of  awns.  No  signifi- 
cant evidence  was  found  that  the  yellow  gene  inhibited  the 
development  of  awns. 

Fraser  (1919)  has  studied  a  cross  between  an  awnless  Sixty 
Day  and  Burt,  the  latter  being  a  variety  of  the  Avena  sterilis 
group.  The  Sixty  Day  parent  produced  bright  yellow  grains 
with  no  awiis.  The  Burt  parent  usually  produced  awns  on  the 
lower  grains  and  frequently  on  the  upper  but  they  show  weak  de- 
velopment. Fraser  classified  awns  as  strong,  intermediate,  and 
weak.  The  strong  awn  is  twisted  at  the  base  and  has  a  sharp 
bend  about  three  eighths  of  the  way  from  the  base  to  the  tip.  It 
is  also  stiff  and  long.  The  intermediate  awn  lacks  the  bend  of  the 
strong  awn  and  is  less  stiff.  It  is  generally  twisted  at  the  base 
and  is  often  curved.  The  weak  awns  vary  greatly  from  almost 
imperceptible  structures  to  weakly  developed  ones.  The  FI 
plants  of  Burt  X  Sixty  Day  were  practically  awnless.  In  Ft 
there  was  a  ratio  of  fully  awned  (awned  like  Burt  or  with  awns 
more  completely  developed)  to  awnless  and  partly  awned  of  1  : 
3.  The  fully  awned  bred  true  in  later  generations.  Results 
substantiated  the  hypothesis  that  Sixty  Day  carried  a  factor  for 
awning  which  was  inhibited  from  development  by  the  yellow 
factor. 

Color  of  Grain  and  Straw. — Color  of  the  lemma  when  ripe 
is  a  character  which  is  easily  affected  by  environment.  Weather 
conditions  at  ripening  are  important  and  greatly  modify  the 
expression  of  inheritance  of  these  color  characters.  With  bright 
sunshine  a  deeper  color  is  developed  than  in  wet,  cloudy  weather. 
Black  or  yellow  grained  varieties  under  unfavorable  environmental 
conditions  are  much  less  intensely  colored.  The  stage  of  matu- 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    93 

rity  at  which  the  grain  is  harvested  or  weathering  after  harvesting 
may  also  modify  these  color  characters. 

The  color  of  the  lemma  of  oats  has  been  classified  as  black, 
brownish  red,  gray,  yellow,  and  white.  Different  varieties, 
likewise,  exhibit  different  intensities  in  the  development  of  a 
particular  color.  In  some  crosses  between  black  and  white  a 
ratio  of  15  blacks  to  1  white  was  obtained  in  F%  (Nilsson-Ehle, 
1909),  while  the  majority  of  crosses  show 3 :1  ratios  (Nilsson-Ehle, 
1909),  (Gaines,  1917).  The  simplest  explanation  is  that  each 
color  character  is  due  to  one  or  more  factors,  each  factor  when 
heterozygous  causing  partial  or  complete  development  of  the 
character. 

Results  of  crosses  show  that  yellow  is  dominant  over  white 
or  partially  so.  There  are,  however,  two  yellow  factors  each 
independently  inherited.  In  a  cross  between  Burt,  which 
produces  yellowish  red  seeds,  and  Sixty  Day,  which  produces 
yellow  seeds,  Frazer  (1919)  obtained  a  ratio  of  48  red,  15  yellow, 
and  1  white  in  F?.  These  results  may  be  explained  by  supposing 
Burt  to  carry  two  color  factors,  R  for  red  and  Y  for  yellow,  and 
Sixty  Day  one  factor,  Yl  for  yellow.  Apparently  .R  produces  reds 
either  when  associated  with  F  or  F1  or  when  alone. 

Gray  is  epistatic  to  yellow  (Nilsson-Ehle,  1909)  (Surface,  1916) 
(Love  and  Craig,  1918c)  but  hypostatic  to  black,  while  black  is 
epistatic  to  all  other  colors  so  far  as  determined.  It  has  been 
tested  for  gray,  yellow,  and  white  but  not  for  brownish  red.  As  a 
rule  the  intensity  of  color  is  not  so  great  when  a  factor  for  a  par- 
ticular color  is  heterozygous  as  when  homozygous. 

The  inheritance  of  a  reddish  straw  color  has  been  shown 
by  Pridham  (1916)  to  behave  as  a  simple  Mendelian  monohybrid. 

Hulled  versus  Hull-less. — The  hull-less  condition  has  been 
made  the  basis  of  one  of  the  species  groups,  Avena  nuda.  Numer- 
ous crosses  between  hulled  and  hull-less  forms  have  given  like 
results.  All  investigators  of  these  crosses  have  obtained  an 
intermediate  condition  in  FI,  with  both  kinds  of  grains,  hulled 
and  hull-less,  borne  in  the  same  panicle.  Ratios  in  F*  of  1  of 
each  of  the  hulled  and  hull-less  forms  to  2  heterozygotes  have 
been  obtained.  The  hulled  and  hull-less  types  breed  true  while 
the  intermediates  again  segregate.  Love  and  McRostie  (1919) 
have  found  considerable  variation  in  the  percentage  of  hulled  and 
hull-less  seeds  in  different  panicles  of  the  same  cross.  Con- 
sistent correlation  was  obtained  between  the  percentage  of  hulled 


94  BREEDING  CROP  PLANTS 

grains  on  heterozygous  F2  plants  and  that  of  hulled  grains  on 
heterozygous  F3  plants.  Some  heterozygous  F2  plants  with  low 
percentages  of  hulled  grains  gave  heterozygous  progeny  with  cor- 
respondingly low  percentages.  A  similar  behavior  was  obtained 
in  the  progeny  of  heterozygous  plants  with  high  percentages  of 
hulled  grains,  while  plants  with  intermediate  percentages  of 
hulled  grains  gave  heterozygous  progeny  with  low,  intermediate, 
and  high  percentages  in  different  plants.  This  suggests  the 
presence  of  a  factor  which  affects  the  percentage  of  hulled  and 
hull-less  grains  of  heterozygous  plants. 

Pubescence. — Cultivated  varieties  of  oats  differ  in  the  amount 
and  in  the  presence  and  absence  of  basal  hairs  on  each  side  of  the 
callus.  In  some  crosses  only  one  factor  is  involved,  in  others  two 
factors.  In  some  crosses  between  parents  which  have  different 
degrees  of  pubescence  there  is  an  increase  in  the  number  of  basal 
hairs,  and  forms  are  obtained  in  F2  which  have  more  pubescence 
than  either  parent,  likewise  forms  which  lack  pubescence.  Cer- 
tain wild  forms  of  Avena  fatua  carry  two  independently  inherited 
factors  for  pubescence  (see  Surface,  1916;  Zinn  and  Surface, 
1917;  Nilsson-Ehle,  1908;  Love  and  Craig,  1918c). 

Characters  of  Base  of  Lower  Grain. — In  wild  forms  of  Avena 
fatua  and  cultivated  forms  of  Avena  sterilis  there  is  a  distinct 
articulation  at  the  base  of  the  lower  grain.  According  to  Surface 
(1916)  this  causes  wild  oats  to  shatter  while  in  cultivated  races  of 
saliva  the  grains  are  not  easily  separated  from  their  base  and  do 
riot  ordinarily  shatter.  The  FI  generation  of  a  cross  between 
A .  fatua  and  Kherson  was  intermediate  as  regards  the  base  of  the 
lower  grain,  but  nearer  the  cultivated  form,  while  the  upper  grain 
had  a  base  similar  to  the  cultivated  parent.  Segregation  in  F2 
g^ave  a  ratio  of  wild,  intermediate,  and  cultivated  of  1:2:1. 
This  leads  to  the  assumption  of  a  single  factor  difference  which 
separates  cultivated  and  wild  in  the  form  of  the  base.  As  has 
been  mentioned,  there  is  strong  association  of  many  other  charac- 
ters and  the  wild  form  of  the  base.  Love  and  Craig  (1918c) 
found  an  indication  of  a  single  factor  difference  for  the  presence 
and  absence  of  basal  articulation  but  found  that  the  yellow  factor 
inhibited  the  development  of  the  wild  or  articulated  base. 

Avena  sterilis  differs  from  other  oat  species  in  having  the  upper 
grain  persistent  to  the  rachilla.  The  base  of  the  lower  grain 
resembles  A.  fatua  in  its  articulation.  In  crosses  between  Burt, 
belonging  to  A.  sterilis,  and  Sixty  Day,  the  FI  was  intermediate 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    95 

and  in  F2  the  articulated  basal  types  could  easily  be  determined. 
These  occurred  in  a  close  approximation  of  the  ratio  of  1  articu- 
lated base  to  3  of  the  intermediate  and  saliva  types  (Fraser, 
1919). 

Open  versus  Side  Panicle. — Nilsson  (1901)  has  used  panicle 
types  and  seed  colors  as  a  chief  means  of  classification.  The 
distinction  between  the  side  and  the  open  panicle  is  easily  made, 
but  the  various  transitional  open  panicled  forms  are  not  easily 
used  in  differentiation.  Nilsson-Ehle  (1908)  has  explained 
crosses  between  an  open-panicled  and  a  side-panicled  variety  on 
the  basis  of  two  main  factor  differences.  Either  factor  when 
homozygous  or  heterozygous  produces  open  panicles.  When 
both  factors  are  homozygous  a  variety  with  an  open  panicle  and 
drooping  branches  is  obtained.  When  the  factors  are  absent  a 
side  panicle  results.  From  crossing  two  open-panicled  forms, 
9-side  forms  were  obtained  out  of  a  total  of  112  plants.  These 
side-panicled  plants  bred  true  while  of  the  103  open-panicled 
plants,  24  again  segregated  giving  both  open-  and  side-panicled 
forms.  The  parental  varieties  have  panicles  with  erect  branches 
while  a  part  of  the  open-panicled  segregates  have  drooping 
branches. 

Resistance  to  Rust. — Parker  (1918)  studied  varietal  resistance 
of  oats  to  stem  rust,  Puccinia  graminis  avence  Erikss.  and  Henn. 
and  to  crown  rust,  Puccinia  lolii  avence  McAlpine.  Crown  rust 
is  a  serious  disease  in  the  South  while  stem  rust  is  more  common 
in  the  North.  Several  varieties  of  the  red  oat  group  of  A. 
sterilis  including  Burt,  proved  resistant  to  crown  rust,  while 
certain  side  oat  strains  of  A.  saliva  orienlalis  belonging  to  the 
White  Russian  group  proved  resistant  to  stem  rust. 

Studies,  of  the  inheritance  of  resistance  to  crown  rust  under 
greenhouse  conditions,  of  crosses  of  Burt  with  Sixty  Day,  A. 
saliva,  showed  segregation  in  F2.  Susceptible  and  resistant 
plants,  as  well  as  various  intermediates,  were  obtained  (Parker, 
1920). 

A  study  of  the  inheritance  of  resistance  to  stem  rust  has 
been  made  at  the  Minnesota  Station  (Garber,  1921).  FI,  F2, 
and  F3  crosses  of  resistant  White  Russian  with  two  susceptible 
varieties  of  A.  saliva,  Victory  and  Minota,  have  been  grown. 
The  preliminary  results  show  that  for  these  crosses  resistance  is  a 
dominant  character,  the  ratio  in  Fz  of  resistant  and  susceptible 
plants  approximating  3:1.  Susceptible  F2  plants  bred  true  to 


96  BREEDING  CROP  PLANTS 

susceptibility  in  F3,  while  resistant  F2  plants  were  of  two  kinds: 
(1)  those  which  produced  only  resistant  progeny  and  (2)  those 
which  segregated,  both  resistant  and  susceptible  plants  being 
obtained. 

Size  Characters. — Nilsson-Ehle  (1908)  made  numerous  stu- 
dies of  inheritance  of  size  characters.  In  a  cross  between  two 
sativa  varieties  which  differ  in  height,  transgressive  segregation 
occurred  in  F2.  Forms  were  selected  and  the  studies  continued 


FIG.  22. — Culms  of  resistant  and  susceptible  varieties  of  oats.  From  left  to 
right:  Victory,  susceptible  to  stem  rust;  a  susceptible  Fz  plant  of  Victory  X 
White  Russian;  a  resistant  Fz  plant  of  Victory  X  White  Russian;  resistant  White 
Russian. 

through  F4  and  F6.  Segregation  was  of  a  complex  nature. 
Transgressive  segregation  also  occurred  in  crosses  involving  leaf 
breadth,  kernel  size,  and  number  of  florets  to  the  spikelet.  The 
results  were  explained  on  the  multiple  factor  hypothesis,  but  the 
actual  factors  involved  could  not  easily  be  determined.  Maturity 
may  be  considered  under  this  heading,  for  it  behaves  in  a  similar 
manner.  From  crossing  early  and  later  maturing  oats',  Caporn 
(1918)  obtained  intermediate  maturity  in  FI  and  segregation 
in  F&  The  author  suggests  that  three  factors  will  quite  satis- 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    97 

factorily  explain  the  results.  Nilsson-Ehle  obtained  trans- 
gressive  segregation  in  Fz  in  a  cross  between  medium  early  and 
late  maturing  varieties.  Progeny  from  112  Fz  plants  were  grown 
in  F3.  Of  these  112  plants,  98  gave  segregating  progeny  for 
maturity  and  14  seemed  to  be  homozygous.  Homozygous 
forms  were  obtained  which  were  earlier  than  the  early  parent 
and  others  which  were  later  than  the  late  parent. 

Linkage  of  Characters. — Association  of  numerous  characters 
in  inheritance  has  been  mentioned  in  the  discussion  of  crosses 
between  the  wild  A.  fatua  and  cultivated  varieties  of  A.  saliva. 
Aside  from  the  general  characters  mentioned,  linkage  has  been 
found  between  the  factor  for  black  color  of  the  lemma  and  one 
of  the  factors  for  pubescence. 

In  crosses  between  Burt,  A.  sterilis,  and  Sixty  Day,  A.  saliva, 
Fraser  (1919)  has  found  that  the  factors  for  the  articulated  base 
of  the  lower  grain,  the  awned  condition,  and  the  production  of 
medium  basal  hairs  were  linked  in  inheritance.  In  the  following 
diagram  A  represents  the  factor  for  awning,  B  for  Burt  base,  and 
C  a  factor  for  the  production  of  medium  basal  hairs. 

0  4.14        5.00 


A  B  C 

The  percentages  of  cross-overs  were  determined  for  F2  and 
F3.  As  has  been  pointed  out,  each  of  the  characters  depends  on  a 
single  factor  for  its  development.  Five  per  cent,  of  cross-overs 
occurred  between  the  factors  for  awning  and  basal  hairs;  4.14 
per  cent,  between  awning  and  the  factor  for  Burt  base,  and  1.79 
per  cent,  between  Burt  base  and  basal  hairs. 

False  Wild  Oats. — False  wild  oats  differ  from  the  cultivated 
varieties  in  the  production  of  heavier  awns,  in  heavy  pubescence, 
and  in  the  basal  articulation.  False  wild  oats  resembling  culti- 
vated varieties  in  color  and  panicle  characters  have  been  found 
by  numerous  investigators.  Nilsson-Ehle  (191  la)  has  reported 
false  wild  oats  in  eleven  pure-line  selections  and  in  two  commer- 
cial varieties  belonging  either  to  A.  saliva  or  A.  saliva  orienlalis. 
A  heterozygous  false  wild  form  was  found  in  the  second  genera- 
tion of  a  cross  between  saliva  varieties.  It  gave  a  ratio  of  1 
cultivated,  2  heterozygous  to  1  false  wild  form.  The  heterozy- 


98  BREEDING  CROP  PLANTS 

gous  forms  are  less  heavily  awned  than  the  false  wild  and  have 
the  fatua  type  of  callus  only  on  the  lower  grain.  Considerable 
difference  of  opinion  is  held  regarding  the  cause  of  the  production 
of  false  wild  oats.  Whether  they  originate  as  a  loss  mutation 
or  through  hybridization  or  both  is  not  yet  determined.  Some 
evidence  for  hybridization  and  some  for  mutation  has  been 
obtained. 


CLASSIFICATION  AND  INHERITANCE  IN  BARLEY 

Students  of  barley  classification  have  frequently  used  density 
and  sterility  of  the  lateral  florets  as  chief  means  of  separating 
the  larger  cultivated  groups.  While  density  is  quite  a  stable 
character,  there  are  gradations  in  the  length  of  the  internode 
from  the  very  lax  to  the  very  dense  spikes  without  any  clear-cut 
differentiation  between  the  mid-dense  and  mid-lax  groups. 
While  density  is  an  important  character  by  means  of  which  to 
differentiate  forms,  it  is  not  very  usable  as  a  chief  means  of 
group  classification.  Harlan  (1918)  has  made  an  interesting 
review  of  barley  classification  studies  and  has  presented  a  new 
grouping  in  which  species  are  made  on  the  basis  of  fertility  of 
the  lateral  florets.  The  following  key  is  taken  from  Harlan 's 
paper : 

All  spikelets  fertile  (six-rowed  barley) 

Lemmas  of  all  florets  awned  or  hooded Hordeum  vulgare  L. 

Lemmas  of  lateral  florets  without  awns  or  hoods . .  .H.  intermedium  Kcke. 

Only  the  central  spikelets  fertile  (two-rowed  barley) 

Lateral  spikelets  consisting  of  outer  glumes,  lemma,  palea,  rachilla,  and 
usually  rudiments  of  sexual  organs H.  distichon  L. 

Lateral  spikelets  reduced  usually  to  only  the  outer  glumes  and  rachilla, 
rarely  more  than  one  flowering  glume  present  and  never  rudiments  of 
sexual  organs H.  deficiens  Steud. 

There  are  several  contrasting  characters  by  means  of  which 
variety  groups  are  made.  Harlan  has  used  the  following  to  dif- 
ferentiate the  variety  groups  belonging  to  each  of  the  four  species 
groups : 

Seeds  hulled;  seeds  naked. 
Lemmas  awned;  lemmas  hooded. 
Seeds  white,  blue,  purple;  seeds  black. 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    99 

In  classifying  the  cultivated  varieties  of  barleys,  the  density 
of  the  spike,  its  shape,  and  the  appearance  of  the  awns  as  well 
as  the  color  of  the  seed,  have  been  used.  Smooth-awned  varie- 
ties are  being  produced  and  it  is  only  a  question  of  time  before 
nearly  all  awned  varieties  will  be  represented  by  both  the  rough 
and  smooth-awned  forms. 

Species  Crosses. — Two  general  results  have  been  obtained 
from  crossing  two-  and  six-rowed  varieties.  The  most  frequent 
result  is  an  intermediate  condition  in  FI  in  which  the  lateral  florets 
are  awned,  but  produce  little  or  no  fruitfulness.  In  F2  a  1  :  2  :  1 
ratio  of  six-rowed,  intermediate,  and  two-rowed  forms  is  obtained. 
Six-rowed  and  two-rowed  forms  breed  true  to  these  respective 
characters  in  later  generations.  Results  of  this  nature  can  easily 
be  explained  on  a  single  main  factor  difference  (Biffen,  19076; 
Gaines,  1917). 

The  intermedium  barleys  have  generally  been  considered  to 
be  of  hybrid  origin.  A  cooperative  study  carried  on  at  the  Min- 
nesota Experiment  Statioa  has  shown  the  probable  origin  of  some 
intermedium  forms  (Harlan  and  Hayes  1920).  In  a  cross  between 
Manchuria,  a  six-rowed  barley,  and  Svanhals,  a  two-rowed 
variety,  the  FI  was  slightly  fruitful  and  produced  intermediate 
developed  awns  on  the  lateral  florets.  In  F2  a  wide  range  of  forms 
was  obtained.  The  genetic  nature  of  the  F2  plants  was  deter- 
mined by  growing  seed  of  each  in  ^3.  From  the  ^3  results  it 
was  possible  to  classify  F2  plants  as  follows: 

1.  Those  that  bred  true  for  the  six-rowed  character. 

2.  Those  that  segregated,  giving  six-rowed,  awned    intermediate  forms 
with  very  high  fruitfulness  of  the  lateral  florets  and  intermedium  forms  in 
a  1:2:1  ratio. 

3.  Intermedium  forms  that  bred  true,  giving  few  or  no  awns  on  lateral 
florets  and  producing  approximately  50  per  cent,  of  barren  lateral  florets. 

4.  Those  that  gave  all  forms  as  in  F?. 

5.  Those  that  produced  intermediates  and  two-rowed  types. 

6.  Those  that  produced  six-rowed,  awned  intermediates  with  little  or  no 
fruitfulness  in  the  lateral  florets  and  two-rowed  forms  in  a  1:2:1  ratio. 

7.  Those  that  bred  true  for  the  two-rowed  condition. 

Results  were  accurately  explained  by  considering  the  Manchu- 
ria parent  to  contain  two  factors,  one  for  six-rowed  and  one  for 
intermedium,  which  was  hypostatic  to  the  six-rowed  factor.  It 


100  BREEDING  CROP  PLANTS 

was  thought  possible  that  minor  modifying  factors  were  some- 
times present  which  influenced  the  degree  of  fruitfulness  of  the 
lateral  florets. 


FIG.  23. — Individual  spikes  of  Fz  generation  of  cross  of  Svanhals  X  Manchuria 
representing  phenotypic  progeny  classes  in  which  the  lemmas  of  the  lateral 
florets  are  rounded  and  awnless.  From  left  to  right:  The  two-rowed  class  which 
will  breed  true  in  F3;  low  fertility  class  which  will  give  two-rowed,  low  fertility 
and  intermedium  in  F3;  intermedium  which  will  breed  true  for  intermedium 
habit  in  /*V  (After  Harlan  and  Hayes,  1920.) 

Crosses  between  intermedium  and  six-rowed  forms  gave 
intermediates  of  high  fruitfulness  in  F\  and  a  ratio  of  six-rowed  to 
intermediates  in  FZ  which  indicated  a  single  factor  difference. 
Intermedium  forms  crossed  with  two-rowed  gave  awnless  forms 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    101 

with  very  low  fruitfulness  in  FI  and  a  ratio  indicating  one  main 
factor  difference  in  F2. 


FIG.  24. — Individual  spikes  of  F2  generation  of  cross  of  Svanhals  X  Manchuria 
representing  the  phenotypic  progeny  classes  in  which  lateral  florets  bear  awns. 
From  left  to  right:  Low  fertility  awned  plant  which  will  give  all  classes  of 
segregates  in  F3  as  in  Fz;  high  fertility  awned  which  will  segregate  into  inter- 
medium, high  fertility  awned  and  six-rowed  in  FS;  six-rowed  which  will  breed 
true  in  Fa.  (After  Harlan  and  Hayes,  1920.) 

Biff  en  (1907b)  found  the  sexless  condition  dominant  in  a  cross 
between  deficiens  and  two-rowed.  Results  from  an  F%  generation 
of  a  similar  cross  grown  at  the  Minnesota  station  indicate  that 
it  is  almost  impossible  to  separate  deficiensr  two-rowed,  and  inter- 


102 


BREEDING  CROP  PLANTS 


mediates  by  inspection.  No  other  strains  except  the  parental 
forms  and  various  grades  of  intermediates  were  obtained. 

These  facts  indicate  that  a  classification  made  on  the  basis 
of  sterility  for  the  species  groups  is  reliable. 

Simple  Mendelian  Characters. — So  far  as  studied  there  are 
several  barley  characters  which  can  be  grouped  according  to 
their  inheritance  and  which  give  simple  Mendelian  ratios.  These 
are  summarized  in  Table  XVI. 


TABLE  XVI. — BARLEY 


CHARACTERS    WHICH   SHOW  SIMPLE   MENDELIAN 
INHERITANCE 


Character  differences                          F\ 

Fi                              Authority 

Awnless  vs.  Hooded  .... 

Awnless 

3  awnless  to  1 

Tschermak  (1901) 

hooded 

Hooded  vs.  Awned  

Intermediate 

3  hooded  to  1 

Tschermak  (1901) 

hooded 

awned 

Thatcher  (1912) 

Rough  vs.  Smooth  Awn 

Toothed 

3  toothed  to  1 

Harlan  (1920) 

smooth 

Black  Palea  vs.  Color-    Black                   3   black   to    1 

Tschermak  (1901) 

less 

colorless 

Biffen  (1907b) 

Purple  Palea  vs.  Color- 

Purple                 3  purple  to   1 

Biffen  (1907b) 

less 

white 

Hulled  vs.  Naked  

Hulled                  3  hulled  to   1 

Thatcher  (1912) 

naked 

Gaines  (1917) 

A  study  of  inheritance  in  barley  was  made  at  the  Minnesota 
Station  in  cooperation  with  the  Office  of  Cereal  Investigations, 
United  States  Department  of  Agriculture.  A  cross  of  Virginia 
Hooded,  a  six-rowed,  hulled,  hooded,  colorless  barley,  with  Jet, 
a  two-rowed,  naked,  awned,  black-glumed  barley  was  studied. 
In  this  cross  there  was  apparently  only  one  factor  difference  be- 
tween two-rowed  and  six-rowed  and  the  intermediate  forms  were 
classed  as  two-rowed,  although  they  could  be  differentiated  from 
true  two-rowed  forms  by  the  presence  of  awns  on  the  lateral 
florets.  The  results  showed  that  these  four  factor  pairs  were 
independently  inherited  and  gave  a  close  approximation  to  ex- 
pectation. Crosses  differing  by  four  independently  inherited, 
sharply  differentiated  factor  pairs  have  not  been  frequently 
presented,  therefore  the  results  are  of  some  interest.  They  are 
as  follows : 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    103 

TABLE  XVII. — INHERITANCE  OF  4  INDEPENDENTLY  INHERITED  MENDELIAN 

CHARACTERS 

EXPECTATION       OBTAINED 

Hooded,  two-rowed,  black,  hulled 129.0  113 

Hooded,  two-rowed,  black,  naked 43.0  43 

Hooded,  two-rowed,  white,  hulled 43 . 0  42 

Hooded,  six-rowed,  black,  hulled 43 . 0  56 

Bearded,  two-rowed,  black,  hulled 43 . 0  45 

Hooded,  two-rowed,  white,  naked 14 . 3  14 

Hooded,  six-rowed,  black,  naked 14.3  15 

Hooded,  six-rowed,  white,  hulled 14.3  14 

Bearded,  two-rowed,  black,  naked .  14 . 3  14 

Bearded,  two-rowed,  white,  hulled 14.3  17 

Bearded,  six-rowed,  black,  hulled 14.3  14 

Hooded,  six-rowed,  white,  naked 4.8  4 

Bearded,  two-rowed,  white,  naked 4.8  6 

Bearded,  six-rowed,  black,  naked 4.8  6 

Bearded,  six-rowed,  white,  hulled 4.8  4 

Bearded,  two-rowed,  white,  naked 1.6  1 

Totals 407.6  408 

Biffen  found  that  there  was  a  correlation  between  the  black 
color  of  the  grain  and  the  color  of  the  palea  in  barley  crosses. 
Two  Japanese  workers,  Miyazawa  (1918)  and  So  (1918),  inde- 
pendently, have  found  xenia  when  white-seeded  varieties  were 
pollinated  with  black-seeded  strains. 

Winter  versus  Spring  Habit. — Fruwirth  (1909)  lists  spring 
forms  as  dominant  over  winter  as  the  usual  mode  of  inheritance. 
Gaines  (1917)  has  obtained  some  winter  forms  from  spring 
crosses.  In  one  cross  he  obtained  18.75  per  cent,  winter  plants 
and  81.25  per  cent,  spring  plants  in  F2.  Results  were  explained 
by  supposing  one  variety  to  carry  a  factor  for  winter  habit  which 
was  prevented  from  expression  by  an  inhibitory  factor.  The 
other  parent  was  considered  to  lack  both  factors. 

Density  of  the  Spike. — Biffen  (1907b)  studied  two  crosses 
between  barleys  which  differ  in  the  length  of  internode  of  the 
spike.  He  found  the  FI  nearly  as  lax  as  the  nutans  parent  and 
obtained  curves  in  F2  which  indicated  that  there  was  one  main 
factor  difference.  Some  of  the  more  dense  F2  segregates  were 
tested  in  Fz.  From  65  plants  so  tested,  55  proved  homozygous 
for  the  dense  condition. 

A  biometrical  study  of  inheritance  of  density1  in  a  number  of 

1  The  average  length  of  internode  in  the  middle  of  the  spike  was  obtained 
by  measuring  the  length  of  10  central  internodes,  in  millimeters,  and  point- 
ing off  one  place. 


104  BREEDING  CROP  PLANTS 

crosses  has  been  made  at  the  Minnesota  Station  in  cooperation 
with  the  Office  of  Cereal  Investigations  (Hayes  and  Harlan, 
1920).  In  a  cross  between  Manchuria  and  Svanhals,  the  FI 
proved  nearly  as  dense  as  the  Svanhals  parent.  Fz  and  F3 
results  showed  these  parents  to  differ  by  one  main  density 
factor. 

Pyramidatum,  a  dense,  six-rowed  form,  was  crossed  with 
Jet,  a  lax  two-rowed  form.  The  average  length  of  internode  of 
Pyramidatum  was  2.11  mm.,  of  Jet  3.92  mm.,  of  theFi,  2.86  mm. 
of  F2,  3.01  mm.  All  forms  were  grown  the  same  year.  The 
Fz  generation  gave  a  continuous  range  of  variability  which 
reached  beyond  the  modal  classes  of  the  parents.  Forms  bred 
true  in  F3  to  densities  which  were  not  widely  different  from  those 
of  the  parents  but  no  homozygous  intermediates  were  obtained. 
Apparently  these  parental  types  differ  by  a  single  main  density 
factor.  There  are  other  minor  factors  which  influence  the 
expression  of  density.  One  such  minor  factor  difference  is 
known.  Some  barleys  have  a  slight  progressive  increase  in  inter- 
node  length  from  the  base  to  the  tip  of  the  spike,  in  others  all 
internodes  have  nearly  the  same  average  length. 

Different  results  were  obtained  in  a  cross  between  Hanna,  a 
lax,  two-rowed  variety,  and  Zeocriton,  a  very  dense  variety. 
The  FZ  gave  somewhat  similar  segregation  as  in  preceding 
crosses,  but  the  ^3  lines  showed  intermediates  as  well  as  extremes 
breeding  true.  Certain  Fs  families  were  as  variable  as  Fz, 
others  were  more  variable  than  the  parents,  and  still  others  were 
homozygous.  Four  possible  modes  of  density  were  found  in 
which  homozygous  segregates  were  obtained,  the  very  dense 
with  means  from  2.1  to  2.3,  the  dense  with  means  ranging  from 
2.8  to  3.2,  the  lax  with  means  ranging  from  3.4  to  3.7,  and  the 
very  lax  with  means  ranging  from  4.2  to  4.3.  If  a  large  number 
of  types  could  have  been  tested,  it  seems  very  reasonable  to  con- 
clude that. homozygous  forms  with  an  almost  continuous  range  in 
average  length  of  internode  from  the  very  dense  to  the  very  lax 
could  have  been  obtained.  Several  main  density  factors  are 
here  involved  together  with  minor  factors. 

The  Barley  Awn  in  Relation  to  Yield.1— The  long  rough 
awn  of  barley  makes  the  crop  very  disagreeable  to  handle. 
Hooded  varieties  have  been  frequently  tried  out  but  have  not 
been  extensively  grown  because  they  do  not  yield  as  well  as 

1  See  HARLAN  and  ANTHONY  (1920). 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    105 

standard  awned  strains.  Likewise,  many  hooded  hybrids  have 
been  produced  but  none  has  proved  satisfactory.  The  facts 
lead  to  the  conclusion  that  "the  awn  is  an  organ  that  is  func- 
tional under  most  conditions,  and  especially  in  those  sections 
where  humid  weather  prevails,  at  ripening  time."  A  study  of  the 
effect  of  the  removal  of  the  awns  on  the  development  of  the  seeds 
of  the  spike  showed  that  spikes  with  awns  removed  (clipped 
spikes)  produce  a  lower  weight  of  dry  matter  at  maturity  than 
normal  spikes.  As  the  seeds  develop  as  rapidly  for  several  days, 
after  the  awn  is  removed,  in  clipped  spikes  as  in  undipped,  the 
difference  in  development  at  maturity  is  not  due  to  the  shock  of 
removing  the  awns.  About  one  week  after  flowering,  the  deposit 
of  dry  matter  in  the  normal  spikes  begins  to  exceed  that  in  the 
clipped  spikes.  This  is  stated  to  be  at  the  time  rapid  starch 
infiltration  begins.  Normal  spikes  at  maturity,  near  Aberdeen, 
Idaho,  have  a  content  of  more  than  30  per  cent,  of  ash  in  the 
awns.  The  rachises  of  the  clipped  spikes  at  maturity  contained 
about  25  per  cent,  more  ash  than  those  of  normal  spikes,  which 
probably  accounts  for  the  greater  tendency  of  clipped  spikes  to 
break. 

These  facts  show  that  under  humid  conditions  there  is  a 
physiological  reason  why  awned  varieties  yield  higher  than 
hooded  or  awnless  varieties.  They  are  given  as  an  illustration 
of  the  value  to  the  plant  breeder  of  a  knowledge  of  the  physiolog- 
ical functions  of  the  various  organs  of  plants. 

Two  methods  of  attack  are  outlined  for  the  barley  breeder: 
(1)  The  use  of  varieties  which  normally  have  a  low  percentage  of 
ash  in  the  rachis  might  make  possible  the  production  of  non- 
shattering  hooded  and  awnless  sorts.  (2)  The  production  of 
smooth-awned  varieties,  which  in  a  large  measure,  would  over- 
come the  objection  to  the  barley  awn. 

The  production  of  high-yielding  smooth-awned  varieties  is  not 
a  difficult  task,  as  has  been  learned  by  cooperative  studies  carried 
on  at  the  Minnesota  Station.  As  smooth  awn  is  a  recessive 
character,  all  that  is  necessary  is  to  cross  high-yielding  toothed 
varieties  with  smooth-awned  sorts,  and  then  select  smooth- 
awned  plants  in  F2.  These  will  breed  true  for  the  smooth-awned 
character.  Numerous  plants  should  be  selected,  as  some  will 
prove  more  valuable  than  others  for  economic  characters  such 
as  yield,  non-shattering  habit,  and  stiffness  of  straw. 


106  BREEDING  CROP  PLANTS 

SOME  RYE  STUDIES 

Wild  rye,  Secale  montanum,  differs  from  cultivated  rye  in  its 
perennial  habit.  Tschermak  (1914)  finds  that  wild  and  culti- 
vated forms  may  be  easily  crossed,  which  indicates  rather  close 
relationship. 

Rye  (see  page  40)  differs  from  the  other  small  grains  in  that 
it  is  cross-pollinated.  Sterility  is  often  obtained  when  self- 
fertilization  is  attempted.  For  this  reason  it  is  not  easy  to 
produce  homozygous  strains  and  therefore  few  inheritance  studies 
have  been  made. 

Xenia  in  rye  was  first  discovered  by  Giltay  in  1893.  It  was 
later  corroborated  by  Von  Rlimker  and  others  (1913,  1914). 
By  continuous  selection,  strains  have  been  produced  which  are 
pure  for  color  differences.  According  to  Von  Riimker,  selection 
for  seven  or  eight  years  was  necessary  in  order  to  isolate  strains 
which  were  homozygous  for  color  of  seed.  He  found  the  color 
to  be  located  in  the  aleurone  layer  just  inside  the  epidermis. 
There  are  numerous  colors  of  rye  which  are  roughly  analogous  to 
the  aleurone  colors  of  corn.  The  inheritance  of  these  colors  has 
not  as  yet  been  intensively  studied.  Von  Riimker  has  isolated 
pure  races  for  greenish  blue,  deep  brown,  and  yellow  seed. 
There  are  also  deep  blue,  light  brown,  and  striped  seed  besides 
other  color  variations.  In  crosses  between  green-  and  yellow- 
seeded  strains  Von  Riimker  found  green  dominant  and  obtained 
a  ratio  of  3  green  to  1  yellow  in  F^. 

There  are  both  spring  and  winter  varieties  of  rye.  The  spring 
habit  appears  to  be  a  dominant  character,  for  Tschermak  (1906) 
obtained  a  ratio  in  F2  of  3  spring  forms  to  1  winter  form. 

Wheat-rye  Hybrids. — Numerous  investigators  (Backhouse, 
1916-1917 ;Leighty,  1915,  1916;  Jesenko,  1911,  1913;  McFadden, 
1917)  have  reported  crosses  between  wheat  and  rye.  In  all 
reported  successful  crosses,  wheat  has  been  used  as  the  female 
parent.  Rye  is  very  winter  hardy  and  as  winter  wheat  is  much 
less  hardy  it  is  only  natural  to  try  to  improve  winter  wheat  by  a 
rye-wheat  cross.  As  a  rule  the  FI  cross  is  self-sterile,  although 
back  crosses  with  the  parents  have  sometimes  been  successful. 
Love  and  Craig  (1919  a)  have  described  a  successful  wheat-rye 
cross,  using  Dawson's  Golden  Chaff  as  the  wheat  parent.  Stud- 
ies have  been  continued  through  F±  and  F$  and  a  number  of 
plants  have  been  obtained  which  exhibit  little  or  no  sterility. 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    107 


These  plants  are  wheat-like  in  spike  and  seed  characters,  yet 
they  resemble  rye  in  some  other  characters.  They  are  now  being 
tested  for  winter  hardiness. 


3  f 


FIG.  25. — Spikes  from  four  t\  plants  of  a  wheat-rye  cross.  Spike  No.  39  is 
much  like  rye  in  regard  to  the  awn  development  and  ciliated  glumes.  Other 
heads  resemble  wheat  more  than  rye.  (After  Love.} 

BUCKWHEAT1 

Buckwheat  belongs  to  the  buckwheat  family  (Polygonacece). 
The  original  home  of  this  plant  was  probably  Asia,  whence  it  was 
introduced  into  Europe  through  Tartary  and  Russia  in  the  middle 
ages.  The  generic  name  of  buckwheat,  Fagopyrum,  comes  from 
the  Latin,  fagus,  beech,  and  the  Greek,  puros,  wheat,  based  on 
the  fact  that  the  seed  of  buckwheat  resembles  the  beechnut. 
The  three  species  of  economic  importance  are  F.  emarginatum, 
F.  tataricum,  and  F.  esculentum.  The  forms  commonly  grown 
in  the  United  States  belong  to  the  last-named  species.  Classifi- 
cation is  based  on  such  characters  as  size,  color,  and  shape  of 
seed;  color  of  growing  stem;  average  height  of  plant;  shape  of 
leaf;  and  flower  characters.  The  flowers  of  buckwheat  are 
dimorphic,  i.e.,  some  have  long  stamens  and  short  styles,  others 
just  the  reverse.  Only  one  kind  of  flower  is  produced  on  the 

1  CARLETON,  1916. 


108  BREEDING  CROP  PLANTS 

same  plant.  According  to  Carleton,  seeds  of  either  form  of 
plant  produce  both  kinds,  and  the  ratio  is  not  influenced  by  soil 
quality.  Dimorphism  facilitates  cross-pollination. 

Breeding  Buckwheat. — Little  attention  has  been  given  to  the 
improvement  of  buckwheat  by  breeding.  It  shows  considerable 
variation,  and  undoubtedly  strains  could  be  isolated  that  would 
surpass  present  commercial  varieties.  Selections  for  yield 
are  being  made  in  the  United  States.  In  Russia  some  work  is 
under  way  to  produce  strains  or  varieties  with  four-faced  seeds. 
These  are  supposed  to  be  more  resistant  to  early  spring  frosts. 

RICE 

Rice  is  thought  by  Carleton  (1916)  to  have  ''originated  some- 
where in  the  region  from  China  to  India  inclusive."  It  has  not 
been  recorded  with  other  cereals  that  were  grown  in  Egypt  in 
ancient  times.  Little  study  has  been  made  regarding  classifica- 
tion and  genetic  relationship  of  the  wild  and  cultivated  species. 

Cultivated  races  are  classified  into  glutinous  and  non-glutin- 
ous groups.  Other  characters  of  importance  in  varietal  classifica- 
tion are  size,  shape,  and  color  of  seed;  color  of  glumes  and  leaf 
sheath;  awned  or  awnless  glumes;  and  length  of  glumes,  whether 
long  or  short.  A  short  summary  of  inheritance  of  some  indi- 
vidual characters  is  of  interest.  (See  Table  XVIII.) 

Inheritance  of  Characters. — The  endosperm  of  rice  is  glutinous 
or  starchy.  The  glutinous  group  is  not  grown  in  the  United 
States  or  generally  in  Europe  as  a  commercial  crop.  On  cooking, 
it  runs  together  into  a  pasty  mass  while  the  seeds  of  common  rice 
keep  their  shape  when  properly  cooked.  The  starch  of  ordinary 
rice  is  replaced  by  a  sort  of  dextrine  in  the  glutinous  varieties. 
Apparently  one  Mendelian  factor  difference  separates  these 
groups.  The  color  of  the  seed  is  also  an  endosperm  character. 
Blue  is  dominant  over  red  and  red  over  white. 

The  inheritance  of  plant  characters  may  be  explained  by  the 
usual  Mendelian  method.  The  ratios  given  show  that  color 
inheritance  may  be  explained  by  one  or  more  factors.  Ikeno 
(1918)  studied  the  inheritance  of  a  number  of  size  characters. 
In  some  cases  dominance  was  obtained  in  FI.  In  other  charac- 
ters the  FI  was  intermediate.  Complex  segregation  occurred  in 
F2  but  with  no  definite  ratios.  Multiple  factors  were  used  to 
explain  the  results. 


CLASSIFICATION  AND  INHERITANCE  OF  SMALL  GRAINS    109 


1        » 

S 

o     g 

S  £ 

£ 

•5" 

i-H 

-3 

2  2 

2 

1 

3      8 

00               Tj« 

S  "8  £ 

CO 

a  *  2 

1—1    ^-H"   CO 

CO 

CO 

s  ^* 

3 

2     1 

Oi      ^    O5 

O5 

O     HJ 

m            O 

O 

Oi 

i-H 

o  2 

.      .     ft 
O  ID    fl 

jg  s 

Ssf! 

d  S   d 

g   fl   g 

1-1   PH   *"" 

1 

M 

Thomp 
Parnell 
Hoshin 

Hoshin 
Parnell 
Hector, 

JS 

1—  1 

d 
1 

Is 

a 

5 

2 

| 

•2 

[3 

1 

1 

1 

• 

0] 

s 

1 

1 

e] 

3 

G 
I 

<N 

"S 

03 

1 

3 
O1 

£ 

a      "3 
w      "3       » 

Jd 

S 

1 

i 

c 

«      "      J 

S 

£ 

2 

£ 

5    2     * 

• 

" 

3 

*     - 

3 

1 

%        %          0 

O 

o 

S            c 

C 

V 

w      2       •* 

•*-* 

fl 

S      .2 

O 

"S 

g      5      "S 
3      »      C 
•     m      to 

*           1 

_.    bO 
^     0 

o> 

If 

<•      +j 
«       » 

<         & 

1 

c 

<< 

r-  « 

r^        W) 

fcfl 

"c 

K         fl          C 

fl           ft 

0 

1—1     r-l 

T-i     tic 

^        4) 

<0 

0     .2      .2 

1  !  1 

O        CQ         O2 

Segregatio 
NT  CHARA( 

•  X 

I 

o 

CO 

3  white  to 
3  short  to 

-2  ^ 
§  ° 

4)    ^ 

&  £ 

CO    OS 

iNTITATIVE 

Complex  s 

Complex  s 

Segregatio 
Improvem 

2 

H 

H 

PL. 

£ 

-2 
.5 

&  £ 

.1 

0) 

s 

i 

1 

S 

* 

S     1> 

W         PH 

g 

c 

1 

03 

j  "°* 

1 
g 
1 

Is 

li 

III 

II  3 

Susceptibility 

•«l! 

£2  * 

&•  GO  **5 

Black 
Green  or  intei 
Red 

Fi  almost  j 
dominant 

\  Fi  intermedia 

Intermediate 

4     • 

.  , 

.  •  , 

'.    '. 

?    ' 

•   bib 

t  : 

:  S 

:^  1   : 

£ 

'•  "3 

5  $ 

!  U    i 
•   i* 

ill: 

Contrasted  charact< 

Character  of  endosperm  
Red  t>s.  white  seed  

Red  vs.  white  seed  

Red  0s.  colorless  awn  
Palea  brown  vs.  colorless  

Palea  colorless  vs.  yellow  

Stigma  purple  rs.  colorless  .  .  . 
Grains  not  readily  falling  vs.  \ 

Susceptibility  to  disease  causi 
sphceria  Cattanei  vs.  immuni 

White  vs.  red  glume  
Long  vs.  short  glume  
Awned  vs.  awnless  glume  .  .  .  . 

Black  vs.  reddish  brown  awn 
Green  vs.  golden  color  of  inne 
Red  vs.  green  leaf  sheath  

High  vs.  low  stature  
Long  vs.  short  panicle  
Thick  PS.  thin  stem  

Amount  of  tillering  
Time  of  appearance  of  first  ps 
Compact  vs.  loose  grain  arran 
Broad  vs.  narrow  leaf  

Time  of  flowering  
Quality  of  grain  and  yield  .  .  . 

110  BREEDING  CROP  PLANTS 

Time  of  culm  formation  was  carefully  studied  by  Hoshino 
(1915),  who  crossed  an  early  with  a  late  variety.  The  parents 
averaged  83.8  and  113.2  days,  respectively,  from  time  of  planting 
to  jointing,  the  parental  average  being  98.5  days,  while  the  FI 
gave  an  average  of  94  days  from  planting  to  jointing.  The  F% 
generation  equalled  the  combined  range  of  the  parents.  Some 
forms  bred  true  to  the  parental  types  in  F3.  One  form  which 
segregated  in  F3  was  much  less  variable  than  the  F2.  This  line 
could  be  explained  by  the  presence  of  a  single  heterozygous  factor 
for  time  of  shooting.  The  author  suggests  that  three  multiple 
factors  will  explain  the  results. 

Kock  (1917)  crossed  Karang  Serang,  an  early  maturing  good 
quality  rice,  with  Skrivimankotti,  a  variety  of  high  yielding 
ability.  Results  were  not  easily  explained  on  a  factor  basis. 
After  seven  years  some  hybrids  showed  considerable  uniformity. 
Improvements  in  quality  and  quantity  of  yield  were  obtained  as 
shown  by  a  comparison  of  the  parents  and  the  better  of  these 
hybrid  lines. 

These  facts  show  that  correct  methods  of  breeding  rice  are 
similar  to  those  of  the  other  small  grains. 


CHAPTER  VIII 
METHODS  OF  BREEDING  SMALL  GRAINS 

The  progeny  test  is  now  recognized  as  the  best  means  of 
determining  the  comparative  productivity  of  varieties  and 
strains.  Vilmorin's  isolation  principle  was  first  used  in  the 
United  States  in  1897  by  Hopkins,  of  Illinois,  for  corn  breeding, 
and  in  1890  by  Hays,  of  Minnesota,  for  small  grains.  Studies 
in  field-plot  technic  and  in  crop  genetics  have  led  to  standard 
methods  of  breeding  self -fertilized  crops. 

One  of  the  important  steps  for  the  breeder  is  to  obtain  a  broad 
knowledge  of  the  crop  plant  with  which  he  is  to  work.  This 
consists  of  a  knowledge  of  the  home  of  the  plant,  its  wild  and 
cultivated  relatives,  the  existing  varieties  and  their  important 
economic  characters.  It  is  also  necessary  to  learn  the  needs  of 
the  crop  for  the  locality  in  which  the  breeder  is  to  work.  The 
importance  of  this  knowledge  can  not  be  over-emphasized. 
After  obtaining  a  fundamental  knowledge  of  the  crop,  the  work 
in  crop  improvement  naturally  falls  under  three  heads:  (1)  In- 
troduction, (2)  Selection,  (3)  Crossing.  Before  taking  these  up, 
attention  will  be  given  to  a  system  for  recording  plant  pedigrees. 

Method  of  Keeping  Continuous  Records. — There  are  numerous 
methods  of  keeping  records  and  as  a  rule  each  investigator 
will  modify  some  general  scheme  to  fit  his  own  particular  needs. 
It  is  also  recognized  that  a  plan  which  might  prove  satisfactory 
for  an  experiment  station  investigator  who  works  only  in  one 
particular  region  might  not  be  at  all  desirable  for  a  federal 
worker  who  has  charge  of  crop  investigations  over  a  wide  area. 

The  Minnesota  plan  has  proved  quite  satisfactory,  although 
it  is  recognized  that  other  methods  of  equal  simplicity  and  value 
have  been  developed  by  other  workers.  It  is  given  only  as 
suggestive  of  the  necessity  of  accurate  records  and  as  one  means 
of  attaining  that  end.  When  a  new  introduction  is  first  brought 
to  Minnesota  it  is  given  a  Minnesota  accession  number  and  the 
history,  source,  and  other  data  are  entered  in  the  number  book 
for  that  crop.  If  the  new  introduction  is  a  pedigreed  form  from 
a  nearby  state  and  seems  promising  it  is  placed  at  once  in  the 

111 


112  BREEDING  CROP  PLANTS 

variety  test.  If  its  value  is  unknown  it  is  handled  in  the  plant- 
breeding  nursery.  The  three  groups,  introductions,  selections, 
and  crosses,  are  given  nursery  class  and  stock  numbers  for  means 
of  identification.  The  year  of  the  first  test  in  Minnesota  is  also 
carried  (except  in  the  case  of  crosses  where  the  year  that  the 
cross  was  made  is  used) ,  together  with  a  series  number  from  1 
to  as  many  forms  as  are  handled  in  the  class  for  the  year  and 
crop  concerned.  The  following  classes  are  used  with  the  sup- 
position that  the  forms  were  first  tested  in  the  nursery  in  1920: 

Class  1-20-1,  1-20-2,  etc Selections. 

Class  II-20-1,  II-20-2,  etc Crosses. 

Class  III-20-1,  III-20-2,  etc New  Introductions. 

Supposing  for  example  20  new  wheat  introductions  were  grown, 
these  would  be  classed  as  III-20-1  to  111-20-20.  All  individual 
plant  selections  are  placed  in  class  I  if  they  are  made  from  com- 
mercial varieties  or  new  introductions.  The  year  that  they  are 
first  placed  in  the  nursery  is  also  carried,  as  well  as  the  series 
number.  These  class  and  series  numbers  are  not  changed  as 
long  as  the  form  is  continued  in  the  nursery  trial. 

Crosses  are  not  given  a  series  number  until  the  strain  gives 
evidence  that  it  is  homozygous.  For  the  first  few  years  the 
method  of  numbering  used  by  the  United  States  Department  of 
Agriculture  is  followed.  Thus  a  cross  made  between  1-14-1  and 
1-14-20  is  labeled  at  the  time  of  crossing  1-14-1  X  1-14-20. 
The  female  parent  is  written  first.  On  growing  this  cross 
in  FI  a  convenient  number  or  letter  is  used.  Later  generations 
for  the  letter  method  would  appear  as  A  for  F\,  A-l  for  Fz, 
A-l-1  to  A-l-200  if  200  plant  selections  were  grown  in  F3. 
As  soon  as  a  cross  is  purified,  that  is,  when  particular  selections 
appear  homozygous,  they  are  placed  in  the  rod-row  test  and  given 
a  series  number;  thus  the  cross  made  in  1918  would  be  labeled  as 
follows: 

First  year,      Class  11-18,  A 

Second  year,  11-18,  A-l 

Third  year,  11-18,  A-l-1  to  A-l-200 

Suppose  A-l-10  and  A-l-50  appear  homozygous  and  look 
promising,  they  would  be  placed  in  the  rod-row  test  and  receive 
series  numbers  as  II-18-1  and  II-18-2. 

Bank  figuring  books  have  been  found  to  be  quite  satisfactory 


METHODS  OF  BREEDING  SMALL  GRAINS  113 

for  the  yearly  field  notes,  a  separate  book  being  used  for  each 
crop.  The  following  illustrates  the  method  of  keeping  records 
for  the  year  1922. 

1921  HEIGHT,  DATE  OTHER 

NAME  N.S.N.  SOUHCE  IN.  HEADING  FIELD 

NOTES 

Turkey  X  Odessa. .     II-18-1        A-l-W 

After  obtaining  yield  and  taking  notes  on  grain  characters,  the 
yearly  results  are  drawn  off  on  8J^  by  11  paper,  summarized,  and 
filed  for  reference  and  further  study.  Only  general  notes  are 
taken,  such  as  date  heading,  date  mature,  height  in  inches, 
per  cent,  lodged,  degree  lodged,  per  cent,  and  kind  of  destructive 
diseases,  botanical  characters,  grain  color,  plumpness  and  quality, 
weight  per  bushel,  and  yield. 

New  Introductions. — By  means  of  new  introductions  the 
breeder  is  enabled  to  obtain  varieties  or  strains  which  have  been 
produced  by  other  breeders,  or  native  varieties  from  the  original 
home  of  the  crop.  There  is  no  value  in  attempting  to  produce 
a  variety  which  is  adapted  to  a  particular  condition  if  the  quali- 
ties desired  are  to  be  found  in  some  variety  already  grown  in 
another  locality  or  country. 

The  United  States  Department  of  Agriculture  has  a  trained 
corps  of  workers  who  are  constantly  introducing  new  plant  sorts 
from  foreign  countries.  At  the  present  time  the  Office  of  Cereal 
Investigations  of  the  Bureau  of  Plant  Industry  acts  as  a  medium 
for  the  introduction  of  new  varieties  of  small  grains.  Through 
cooperation  with  this  office,  promising  new  introductions  are 
being  tested  in  localities  to  which  they  seem  adapted. 

In  small  grains  no  conclusion  can  be  drawn  from  the  first- 
year  test  of  a  new  introduction  obtained  from  a  widely  different 
climate.  Often  the  seed  does  not  give  a  high  percentage  of 
germination  or  for  some  other  reason  the  results  secured  are  not 
even  indicative  of  the  value  of  the  introduction.  The  first  year 
the  different  introductions  may  well  be  grown  in  short  rows. 
The  following  year  a  rod-row  of  each  new  introduction  may  be 
grown  as  a  part  of  the  regular  crop  breeding  row  trials,  and  yield 
and  other  characters  determined.  Those  which  are  at  all  promis- 
ing by  this  test  may  then  be  placed  in  the  regular  row  trials 
and  handled  in  the  same  manner  as  pure-line  strains.  After 
two  or  three  years  those  introductions  which  give  results  of 
promise  will  be  used  as  a  basis  for  individual  plant  selection, 
providing  the  introduction  was  not  already  a  pure-line. 


114  BREEDING  CROP  PLANTS 

Selection. — The  plant-selection  method  is  used  for  the  purpose 
of  isolating  the  best  possible  pedigreed  strain  of  a  commercial 
variety.  If  the  variety  is  of  considerable  value  a  large  number  of 
individuals  (500  to  1,000)  may  be  selected.  Often  a  smaller 
number  is  all  that  the  breeder  can  afford  to  test.  The  number 
chosen  will  depend  on  the  productive  capacity  of  the  commercial 
variety  or  new  introduction  which  is  used  as  the  basis  of  selection. 
Plant  selections  are  grown  in  short  rows  the  first  year,  the  same 
number  of  seeds  being  placed  in  each  row. 

Two  general  methods  have  been  rather  widely  adopted  for  the 
initial  head-selection  plot.  In  either  method  the  same  number 
of  seeds  is  placed  in  each  row.  The  difference  lies  in  the  spacing 
of  the  seeds.  Some  prefer  to  place  the  seeds  approximately  the 
same  distance  apart  in  the  row  and  at  sufficient  distance  (2%  to 
3  inches)  that  the  plants  can  be  separately  observed.  Others 
scatter  the  seeds  in  short  rows,  placing  them  so  close  together 
that  individual  plants  cannot  be  differentiated  at  maturity.  The 
latter  method  more  nearly  approximates  the  rod-row  plan  and 
needs  less  room.  In  either  case  the  rows  are  usually  a  foot  apart. 

The  field,  after  being  carefully  harrowed,  is  raked  by  hand,  if 
necessary.  It  is  then  marked  out  by  the  use  of  a  sled  marker, 
from  7  to  12  rows  being  marked  at  a  time.  The  rows  are  opened 
with  a  wheel  hoe  and  covered  either  with  it  or  a  rake  or  a  hand 
drag  with  numerous  iron  teeth. 

Those  selections  which  by  field  inspection  seem  to  be  of  inferior 
vigor,  to  have  weak  straw  or  other  undesirable  characters,  are 
eliminated  before  harvesting.  A  few  others  are  discarded  on 
the  basis  of  yield,  although  the  experimental  error  in  a  yield 
comparison  of  this  kind  is  much  too  large  to  justify  rejection. 
The  following  year  each  selection  may  be  grown,  if  sufficient 
seed  is  available,  in  three  systematically  distributed  18-foot 
rows,  1  foot  being  removed  from  each  end  of  every  row  before 
harvesting. 

According  to  Love  and  Craig  (1918a),  J.  B.  Norton,  of  the 
United  States  Department  of  Agriculture,  was  the  first  to  put 
the  rod-row  method  into  general  use.  By  varying  the  length 
of  the  row  and  obtaining  the  yield  in  grams  it  is  possible  to  con- 
vert yields  into  bushels  per  acre  by  multiplying  by  a  simple 
conversion  factor.  If  the  length  of  oat  rows  harvested  is  15  ft. 
and  the  yield  is  obtained  in  grams,  the  yield  per  acre  in  bushels 
may  be  obtained  by  multiplying  by  0.2.  For  wheat  and  barley, 


METHODS  OF  BREEDING  SMALL  GRAINS  115 

if  the  rows  harvested  are  16  and  20  ft.  long,  respectively,  the 
conversion  factor  will  be  0.1. 

The  rod-rows  are  about  twice  as  far  apart  as  the  rows  made  by 
a  field  grain  drill.  As  from  one  and  one-half  to  two  times  as  much 
seed  is  planted  per  nursery  row  as  under  field  planting,  the  rate 
of  seeding  per  acre  does  not  differ  materially  in  the  two  methods. 
These  row  trials  have  been  shown  to  give  results  similar  to  those 
from  field  tests,  although  the  average  yield  of  the  crop  is  not 
the  same  (Montgomery,  1913;  Love  and  Craig,  1918a). 

As  has  been  previously  noted, 'there  are  two  general  methods 
of  work,  i.  e.,  the  use  of  single-  and  three-row  plots.  Three-row 
plots  in  which  the  central  row  only  is  used  to  secure  yield  are  de- 
sirable as  they  help  to  control  mixtures  at  planting  and  harvest- 
ing time,  overcome  competition  between  nearby  varieties  and  help 
in  obtaining  more  dependable  data  on  lodging.  They  require 
more  land  and  the  cost  is  somewhat  greater  for  planting  and 
cultivating.  In  sections  where  soil  heterogeneity  is  very  great 
it  is  possible  that  the  use  of  single-row  plots  and  numerous  repli- 
cations may  be  somewhat  better  than  three-row  plots  and  fewer 
replications.  On  land  that  is  well  suited  for  field  plot  work  the 
use  of  three-row  plots  and  three  replications  is  advised. 

After  a  strain  has  been  grown  for  three  years  it  may  well  be 
removed  from  the  row-yield  trial  and  either  increased  if  it  shows 
promise  or  discarded  if  it  appears  to  be  of  no  value.  At  Cornell 
new  sorts  are  introduced  to  the  farmers  for  trial  directly  from  the 
rod-rows.  In  many  cases  the  new  sort  is  finally  tried  in  variety 
plots  planted  by  the  usual  field-plot  method.  This  gives  an 
expression  of  yield  under  normal  methods  of  planting  and  favor- 
able field  conditions. 

Summary  of  Methods  of  Selection. — 1.  Determination  of  the 
varieties  which  possess  economic  possibilities.  These  may  be 
commercial  varieties  or  new  introductions. 

2.  Head  selection  of  these  promising  varieties. 

3.  Test  of  head  selections  in  plant-rows.    The  very  undesirable 
strains  are  eliminated  in  the  field  by  inspection.     A  few  may  be 
discarded  on  the  basis  of  yield  or  seed  characters. 

4.  Yield  determinations  of  the  selections,  using  three  plots  of 
a  single  row  each,  systematically  replicated,  if  seed  is  available. 

5.  Continuation  of  the  row  test.     When  land  is  well  suited  it 
is  believed  that  four  systematically  distributed  plots  of  three 
rows  each  will  give  reliable  results.     Possibly  the  arrangement 


116  BREEDING  CROP  PLANTS 

of  selections  of  like  nature  together,  the  use  of  single  rows 
and  more  replications,  may  be  desirable  under  certain  condi- 
tions. 

6.  Computation  of  a  probable  error  for  the  method  of  test. 
The  use  of  this  probable  error  as  a  means  of  determining  signifi- 
cant differences. 

7.  Increase  of  the  better  selections  and  either  a  trial  by  careful 
farmers  or  a  further  test  in  field  variety  plots  followed  by  distri- 
bution of  the  better  strains.     If  placed  in  field  variety  plots, 
borders  should  be  removed  and  each  variety  tested  in  repli- 
cated plots.     Probable  errors  should  be  obtained  and  used  as  in 
the  row  trials. 

From  five  to  eight  years'  time  is  needed  before  the  new 
selection  is  introduced  to  the  farmer. 

Crossing. — The  improvement  of  commercial  varieties  of  self 
fertilized  small  grains  by  the  head  or  plant  method  of  selection 
is  a  very  easy  process,  although  several  years  are  required  to  do 
the  work.  The  production  of  new  forms  by  crossing  is  not  so 
simple.  A  standard  plan  of  attack  has  been  developed  which 
is  the  application  of  the  Mendelian  method. 

The  first  step  is  the  initial  cross.  Promiscuous  crossing  is  not 
advised,  but  each  cross  should  be  the  result  of  a  determination 
of  parents  which  most  nearly  approach  the  ideals  in  mind.  By 
recombination  of  characters  there  is  the  possibility  of  obtaining 
a  sort  which  is  more  desirable. 

The  FI  generation  is  grown  so  that  each  plant  has  space  for 
individual  development.  A  knowledge  of  the  inheritance  of 
characters  allows  those  plants  which  are  not  crosses  to  be  elimin- 
ated in  FI.  The  F2  generation  plots  should  be  as  large  as  can  be 
studied  and  each  plant  grown  with  enough  free  space  that  it  may 
be  examined.  Numerous  selections  of  plants  which  have  de- 
sirable field  and  seed  characters  should  be  made.  Each  of  these 
Fz  plants  selected  should  be  grown  in  an  individual  progeny  plot  in 
F3  and  individual  plant  notes  taken.  Selection  of  desirable 
plants  should  be  continued  in  later  generations.  When  plots 
show  apparently  uniform  progeny  of  a  desirable  sort,  the  strain 
should  be  included  in  the  rod-row  tests  and  compared  with 
standard  varieties. 

Knowledge  of  the  results  of  continued  self-fertilization  in 
generations  following  a  cross,  shows  the  reliability  of  another 
method  which  was  first  used  at  Svalof,  Sweden  (Babcock  and 


METHODS  OF  BREEDING  SMALL  GRAINS  117 

Clausen,  1918)  and  is  now  being  tried  by  other  investigators.  It 
consists  of  growing  a  bulk  plot  of  the  cross  for  several  generations. 
At  the  end  of  from  six  to  ten  years,  head  selections  may  be  made 
with  the  knowledge  that  a  large  part  of  these  selections  will  breed 
true.  The  adoption  of  this  plan  will  in  a  large  measure  do  away 
with  the  technic  of  studying  individual  plants  in  a  heterozygous 
population.  It  is  desirable  for  those  workers  who  would  like  to 
use  crossing  methods  but  who  do  not  have  time  for  individual 
plant  studies.  It  is  not  so  rapid  as  the  Mendelian  method. 

Technic  of  Harvesting,  Thrashing,  Etc. — Slight  variations  in 
methods  are  used  by  different  workers.  At  Cornell  rows  of 
like  kind  are  taken  to  the  thrashing  shed  and  hung  head  down 
until  thrashed.  At  the  Minnesota  Station  the  straw  is  cut  near 
the  base,  the  bundles  tied  with  the  stake,  label  near  the  bottom, 
and  the  heads  wrapped  with  a  cheese-cloth  covering.  Bundles  of 
the  same  selection  are  then  tied  upright  to  a  stake  and  later  taken 
to  the  thrashing  shed  when  needed.  The  row  trials  at  the  sub- 
stations are  harvested  by  cutting  off  the  heads.  These  are  then 
put  into  cloth  sacks  and  shipped  to  the  Central  Station. 

Several  machines  which  can  be  cleaned  easily  have  been  devised 
for  thrashing.  The  chief  requisites  of  a  machine  to  be  used  for 
experimental  purposes  are  that  it  be  easily  cleaned  and  that  so  far 
as  possible  there  be  no  ledges  or  ridges  upon  which  seeds  may 
lodge.  The  alternate  thrashing  of  different  nursery  crops  is  a 
desirable  procedure.  Each  of  the  plots  of  one  strain  of  wheat  may 
be  thrashed  separately  in  rotation  and  then  a  strain  of  oats  may 
be  thrashed  in  the  same  way.  At  the  Minnesota  Experiment 
Station  winter  wheat  is  thrashed  alternately  with  barley  and 
spring  wheat  with  oats.  This  plan  helps  materially  to  reduce 
the  roguing  of  accidental  mixtures  from  the  plots. 

Various  machines  have  been  made  to  assist  in  individual  head 
and  plant  thrashing.  A  machine  constructed  by  H.  W.  Teeter, 
of  the  Department  of  Plant  Breeding  at  Cornell  (Love  and 
Craig,  1918a),  is  very  satisfactory.  As  no  screen  or  fan  is  used, 
all  seeds  are  saved.  After  thrashing,  the  seed  is  passed  through 
a  wind  blast.  This  machine  is  so  arranged  that  mixtures  may 
be  avoided. 


CHAPTER  IX 

SOME  RESULTS  OF  SELECTION  WITH  SELF- 
FERTILIZED  CROPS 

In  its  broadest  sense,  selection  is  really  at  the  basis  of  all  animal 
or  plant  improvement  by  breeding.  Evidence  accumulated  by 
early  plant  breeders  indicated  to  them  that  selection  of  the  most 
desirable  plants  for  seed  was  highly  profitable,  irrespective  of 
whether  the  plants  were  naturally  cross-fertilized  or  self-fertil- 
ized. Darwin  believed  that  the  mean  type  of  any  population 
could  be  changed  by  a  plus  or  minus  selection.  It  was  left  for 
Johannsen  (1903)  to  point  out  the  true  significance  of  selection 
within  a  naturally  self -fertilized  crop. 

Before  discussing  Johannsen 's  pure-line  concept  and  its  rela- 
tion to  the  improvement  of  self-fertilized  crops  by  selection,  a 
brief  survey  of  early  work  on  improvement  of  naturally  self- 
fertilized  cereals  is  desirable. 

EARLY    INVESTIGATORS    IN    SELECTION    OF     SELF-FERTILIZED 

CEREALS 

John  Le  Couteur  and  Patrick  Shirreff  were  first  to  use  the  prog- 
eny test  in  making  selections.  The  former  did  considerable 
work  with  wheat.  In  the  early  part  of  the  nineteenth  century 
he  grew  what  he  supposed  to  be  a  uniform  variety.  Professor 
La  Gasca,  of  the  University  of  Madrid,  upon  inspecting  Le  Cou- 
teur 's  wheat  in  the  field  pointed  out  no  less  than  23  distinct 
forms.  This  observation  led  the  latter  to  make  a  collection  of 
150  varieties.  Le  Couteur  simply  took  it  for  granted  that  the 
progeny  of  any  one  individual  would  breed  true.  Patrick 
Shirreff,  another  breeder  of  cereals,  who  lived  in  the  middle  of  the 
nineteenth  century,  worked  along  somewhat  different  lines.  He 
searched  for  the  exceptional  plant  to  start  a  new  variety,  and 
discovered  seven  such  varieties. 

Frederic  F.  Hallett  also  followed  rigid  selection  of  individual 
plants  in  his  wheat  breeding.  Furthermore,  he  proceeded  on  the 
theory  that  the  selection  of  the  best  spike  on  the  plant  and  the 

118 


RESULTS  OF  SELECTION  WITH  SELF-FERTILIZED  CROPS     119 

best  seed  on  the  spike  would  yield  correspondingly  the  best  plant. 
Le  Couteur  and  Shirreff  placed  all  the  emphasis  on  the  original 
plant  selection,  while  Hallett  believed  he  could  improve  the  prog- 
eny of  an  individual  plant  by  further  selection.  Needless  to  say, 
Hallett  made  no  progress  after  the  initial  selection.  A  number  of 
his  improved  varieties  were  introduced  and  widely  grown. 

Louis  Leveque  de  Vilmorin  formulated  a  breeding  principle 
as  a  result  of  a  series  of  experiments  performed  by  himself  and 
his  father  which  was  published  in  monograph  form  (1852). 
These  early  studies  were  carried  on  with  vegetables  and  the  con- 
clusion was  reached  that  the  only  way  to  determine  the  breeding 
value  of  a  plant  was  to  grow  and  examine  its  progeny.  Much 
study  was  made  by  the  younger  Vilmorin  with  the  sugar  beet. 
This  is  not  a  self-fertilized  plant,  but  the  principles  learned 
have  a  direct  bearing  on  selection  with  self -fertilized  crops.  In 
the  first  few  years  the  problem  of  determining  the  sugar  content  of 
mother  beets  without  injury  to  the  roots  received  particular 
attention.  Weighing  a  small  ingot  of  silver  in  the  juice  extracted 
from  a  small  piece  of  root  was  found  to  be  an  accurate  method  of 
determining  density  and  thus  sugar  content.  Roots  of  similar 
sugar  content  were  then  used  as  mother  plants  and  their  breeding 
nature  determined.  Some  gave  progeny  with  high  sugar  content 
without  pronounced  variability;  other  mother  plants  gave  varia- 
able  progeny  some  of  which  were  high  in  sugar  content  and  others 
much  lower,  while  some  mother  beets  produced  progeny  of  such 
inferior  sugar  content  that  all  were  immediately  discarded. 
Later  the  sugar  content  was  determined  by  means  of  polarized 
light  (Babcock  and  Clausen,  1918).  As  an  example  of  his 
results  may  be  mentioned  a  strain  of  beets  which,  after  three 
years'  selection,  gave  juice  with  an  average  density  of  1.087 
while  unselected  seed  grown  in  the  same  field  gave  an  average 
density  of  only  1.042.  Andre  Leveque  de  Vilmorin  produced  a 
desirable  cultivated  form  of  carrot  by  three  years  of  selection  from 
wild  forms.  Louis  de  Vilmorin  also  made  a  collection  of  wheats 
and  other  grains  from  all  parts  of  the  world.  After  50  years 
of  selection  within  isolated  lines  of  wheat,  no  notable  change 
was  observed  (Hagedoorn,  A.  L.  and  A.  C.,  1914). 

Willet  M.  Hays,  formerly  of  the  Minnesota  Experiment  Station, 
was  the  first  in  America  to  adopt  the  "Vilmorin  method" 
for  small  grains.  In  1891  he  introduced  what  is  known  as  the 
centgener  method  of  grain  breeding  (Hays  and  Boss,  1899). 


120  BREEDING  CROP  PLANTS 

Briefly,  it  consisted  of  growing  and  harvesting  a  100-plant  plot 
from  each  plant.  Selection  was  continued  the  following  year. 
The  selections  of  most  promise  were  increased  and  given  exten- 
sive trials  by  farmers.  By  this  method  new  forms  of  superior 
value  were  discovered. 

The  pure-line  method  of  breeding  self-fertilized  crops  was 
independently  discovered  and  later  adopted  (1891)  by  the  Svalof 
experiment  station  in  Sweden.  The  director  of  the  station,  H. 
Nilsson,  was  led  to  its  adoption  by  the  accidental  discovery  that 
only  those  plots  planted  with  seed  coming  from  a  single  plant 
exhibited  uniformity  (Newman,  1912).  DeVries  (1907)  says: 

11  To  this  accidental  circumstance,  combined  with  the  exact  scientific 
method  of  keeping  extensive  records,  the  discovery  of  the  cause  of 
the  diversity  of  the  cultures  was  due.  For  precisely  those  cultures 
which  were  derived  from  one  ear  only  were  found  to  be  pure  and  uniform, 
all  others  offering  to  the  eye  a  more  or  less  motley  assemblage  of  forms." 

The  fact  that  many  of  the  agricultural  varieties  grown  in 
Sweden  at  the  present  time  are  the  result  of  this  method  of  breed- 
ing is  sufficient  evidence  of  its  success. 

In  addition  to  individual  plant  selection,  the  older  mass 
selection  is'  sometimes  used  with  self -fertilized  crops.  Mass 
selection  is  the  selection  of  a  group  of  individuals  which  seem  to 
embody  the  desired  characters.  No  attempt  is  made  to  grow 
the  offspring  of  the  different  individuals  separately  and  hence  a 
pure-line  study  is  impossible.  In  spite  of  this  fact,  mass  selection 
sometimes  has  a  place  in  correct  breeding.  For  example,  it  may 
be  advantageous  to  let  nature  eliminate  non-hardy  forms  of  a 
winter  wheat  variety  before  beginning  a  study  of  individual 
plant  progenies. 

SELECTION  WITHIN  A  PURE  LINE 

Early  in  the  twentieth  century  Johannsen  (1903,  1913)  began 
his  famous  experiments  with  beans  and  barley  which  resulted  in 
the  discovery  of  facts  which  led  to  the  development  of  the  pure- 
line  theory.  Johannsen  found  that  selection  within  a  pure  line 
was  futile.  Table  XIX  is  typical  of  what  he  obtained  by  selection 
within  each  of  19  different  pure  lines  of  beans. 

Since  Johannsen  announced  his  pure-line  concept,  several 
investigators  working  with  other  crops  and  other  characters  have 
verified  his  conclusions. 


RESULTS  OF  SELECTION  WITH  SELF-FERTILIZED  CROPS     121 


TABLE  XIX. — SELECTION  EFFECT  DURING  Six  GENERATIONS  IN  LINE  I 
OF  PRINCESS  BEANS 


Harvest 

Total 
num- 

Mean weight  of 
mother  beans  of 
the  select  strains 

Differ- 
ence, 

Mean  weight  of  progeny 
seeds  of  select  strains 

Difference, 

years 

ber  of 

B        A 

beans 

A-minus 

B  -plus 

A-minus 

B  -plus 

1902 

145 

60 

70 

10 

63.15  +  1.02 

64.85  +  0.76 

+  1.70±1.27 

1903 

252 

55 

80 

25 

75.  19  ±1.01 

70.88±0.89 

-4.31±1.35 

1904 

711 

50 

87 

37 

54.59±0.44 

56.68±0.36 

+  2.09±0.57 

1905 

654 

43 

73 

30 

63.55  +  0.56 

63.64  +  0.41 

+0.09±0.69 

1906 

384 

46 

84 

38 

74.38  +  0.81 

73.00±0.72 

-1.38  ±1.08 

1907 

379 

56 

81 

25 

69.07  +  0.79 

67.66  +  0.75 

-1.41  +  1.09 

Fruwirth  (1917)  made  selections  within  a  pure  line  of  each  of 
the  following:  lentil  (Lens  esculenla),  vetch  (Vicia  saliva), 
snap  bean  (Phaseolus  vulgaris),  field  pea  (Pisum  arvense),  and 
white  mustard  (Sinapis  alba),  but  failed  to  change  significantly 
the  mean  of  the  character  subjected  to  selection.  In  other 
words,  the  genotype  was  not  altered.  Fruwirth  also  conducted 
experiments  within  pure  lines  of  oats.  He  selected  for  number 
and  length  of  hairs  on  the  lower  grain  in  addition  to  selecting 
for  percentage  of  two-grained  spikelets  per  plant.  The  work  was 
carried  on  from  1906  to  1915  without  effecting  permanent  altera- 
tion in  the  hereditary  complex.  Table  XX  taken  from  Fruwirth, 
illustrates  a  typical  case. 

TABLE  XX. — SELECTION  FOR  PERCENTAGE  OF  BRISTLING  IN  OATS 


Minus  selection 


Plus  selection 


vr       _ 

Per  cent,  of  bristling  of                                   Per  cent  of  bristling  of 

Progeny 

Progeny 

Mean 

S.D.                                       Mean 

S.D. 

1907 

i 

5.11+0.682 

1.68±0.482 

1908 

2.5 

5.47  +  1.37 

4.32+0.97 

4.8 

4.05+0.882 

2.78+0.622 

1909 

0 

4.70  +  1.03 

3.24+0.72 

9.2 

4.75±0.99 

3.12±0.69 

1910 

0.67 

2.94+2.36 

11.80  +  1.66 

10.0 

8.46  +  1.61 

8.06  +  1.14 

1911 

0 

0.14±0.07 

0.33+0.05 

21.9 

8.  88  ±1.47 

5.50  +  1.04 

1912 

0 

0.93+0.22 

0.85+0.12 

18.7 

1.02+0.35 

1.65±0.25 

1913 

0 

1.20±0.33 

1.67±0.24 

5.1 

3.58±0.98 

4.18±0.69 

1914 

0 

0.1/4+0.09 

0.44+0.06 

13.0 

0.64+0.25 

1.25±0.17 

1915 

0 

2.65+0.46 

2.32+0.33 

5.7 

3.14+0.55  i2.  74+0.  39 

i             i                       ! 

1  Practically  no  bristles. 

2  Mean  error. 


122 


BREEDING  CROP  PLANTS 


In  the  above  table  mean  error  is  used  instead  of  probable  error 
(mean  error  X  0.6745  =  probable  error).  The  means,  both  in  the 
minus  and  in  the  plus  direction,  show  no  effect  of  continuous 
selection. 

In  1914  Hutcheson  published  the  results  of  13  years  of  con- 
tinuous selection  in  wheat  carried  on  at  the  Minnesota  Station. 
Here  again  no  significant  effects  of  selection  are  found.  Table  XXI 
presents  a  comparison  of  the  yields  for  the  first  five-year  period 
with  those  of  the  last  five-year  period. 

TABLE  XXI. — COMPARISON  OF  AVERAGE  YIELD  PER  PLANT  IN  GRAMS  OF 

FIRST   FIVE-YEAR   PERIOD   WITH    THOSE   OF  LAST  FIVE-YEAR 

PERIOD  IN  CONTINUOUS  SELECTION  OF  WHEAT 


Variety 

First  five-year  period 

Last  five-year  period 

Hedgrow                

2.67 

2.34 

Russian                                  

1.99 

2.18 

Speltz       

2.51 

2.40 

Kainoiiska,        '          

2.01 

1.97 

Polish  1 

1.54 

1.61 

Polish  2                    

1.62 

1.31 

Average . . . 


2.06 


1.97 


In  the  tobacco  breeding  work  of  the  Connecticut  Experiment 
Station  (Hayes,  1913b)  the  inheritance  of  number  of  leaves  was 

TABLE  XXII. — NUMBER  OF  LEAVES  OF  SUMATRA,  403;  BROADLEAF,  401; 
HAVANA,  402;  AND  CUBAN,  405 


» 

Progeny 

Number 

Year 
grown 

Leaves 
of  parent 

Range 

of  varia- 

Total 

Average 

c.v. 

tion 

403 

1910 

24-31 

150 

28.2±0.08 

5.27±0.21 

403-1 

1911 

29 

23-31 

125 

26.5  +  0.11 

6.64  +  0.28 

403-1-2 

1912 

29 

21-32 

151 

26.2±0.12 

8.28  +  0.32 

401 

1910 

17-22 

150 

19.2+0.05 

5.00±0.19 

401-1 

1911 

20 

16-22 

108 

19.1+0.08 

6.54+0.30 

401-1-1 

1912 

22 

17-23 

145 

19.9±0.07 

6.03+0.24 

405 

1910 

16-25 

150 

19.9+0.08 

7.53+0.28 

405-1 

1911 

21 

18-23 

124 

20.6+0.07 

5.29±0.23 

405-1-1 

1912 

23 

17-25 

150 

20.  9  ±0.07 

6.17±0.24 

402 

1910 

17-24 

150 

19.8  +  0.07 

6.98  +  0.27 

402-1 

1911 

20 

16-25 

143 

20.3+0.10 

8.87  +  0.35 

402-1-1 

1912 

20 

17-22 

150 

19.4  +  0.05 

4.59±0.18 

RESULTS  OF  SELECTION  WITH  SELF-FERTILIZED  CROPS     123 


studied.  •  The  parental  forms  were  grown  with  the  hybrids  for 
comparison.  Although  tobacco  is  naturally  self-fertilized,  the 
plants  were  bagged  to  insure  self-fertilization.  The  behavior 
of  the  parental  forms  selected  in  a  plus  direction  is  shown.  It  is 
obvious  from  the  data  presented  that  tobacco,  like  other  self- 
fertilized  crops,  does  not  respond  to  selection  within  a  pure  line; 
at  least  not  to  a  degree  which  would  encourage  the  plant  breeder 
to  use  this  method  of  seeking  improvement.  (See  Table  XXII.) 
Love  and  Craig  (1918b)  recently  reported  on  the  effect  of 
selection  for  height  of  plant  within  a  pure  line  of  oats.  No  evi- 
dence of  selective  effect  was  obtained,  as  is  shown  in  Table  XXIII. 

TABLE  XXIII. — SELECTION  FOR  HEIGHT  WITHIN  A  PURE  LINE  OF  OATS 


Average  height  of  parents 


Average  height  of  offspring 


Year 

selected,   in  cm. 

produced,  in  cm. 

Tall  line 

Short  line 

Tall  line 

Short  line 

1913 

1914 
1915 
1916 

Average.  . 

85.8 

86.9 
94.9 
97.1 

58.8 

60.4 
67.8 
74.9 

74.2 

82.6 
89.4 
95.9 

75.7 

82.9 
88.8 
94.5 

91.2                     65.5 

85.5 

85.5 

An  average  difference  of  25.7  cm.  in  height  of  plant  between 
the  parent  forms  chosen,  failed  to  change  the  genotype. 

One  of  the  old  mooted  questions  among  investigators  of  field 
crops  was  the  relation  between  the  weight  of  seed  planted  and  the 
resultant  yield.  Earlier  workers  adhered  to  the  belief  that  the 
selection  of  large  seed  would  give  increased  yield.  In  a  pure  line 
of  a  self-fertilized  crop,  heavier  seeds  possess  larger  endosperms 
a'nd  consequently  contain  more  stored  food  material  for  the  young 
plantlet  than  the  smaller  seeds.  It  seems  that  it  would  be  pos- 
sible to  have  the  environment  during  the  germination  period 
such  that  the  larger  seeds  would  have  an  advantage  over  the 
smaller  ones.  The  important  fact  to  bear  in  mind,  however,  is 
that  all  seeds  of  the  same  pure  line  have  the  same  inheritance. 

Some  work  has  been  done  (Arny  and  Garber,  1918)  on  the 
relation  between  size  of  seed  planted  and  resultant  yield  in 
Marquis  wheat.  The  seeds  were  individually  spaced  4  in.  apart. 
The  relation  between  the  weight  of  the  seed  in  milligrams,  and 
the  resultant  yield  in  decigrams  was  expressed  by  means  of  a 


124 


BREEDING  CROP  PLANTS 


correlation  coefficient.  The  coefficients  for  the  years  1914, 
1915,  1916,  and  1917  were  0.143  ±  O.C38,  0.088  ±  0.028,  0.445  ± 
0.020,  and  0.478  ±  0.024,  respectively.  In  this  investigation 
each  plant  was  given  the  same  space  for  individual  development. 
The  results  show  that  under  these  conditions  relatively  large 
amounts  of  stored  plant  food  in  the  germinating  seed  may  or 
may  not  give  the  resultant  plants  an  advantage,  depending  on 
environmental  influences  other  than  the  amount  of  endosperm. 

Several  investigators  have  attacked  this  problem  from  a  prac- 
tical viewpoint.  Seeds  were  separated  into  light,  medium,  and 
heavy  by  means  of  a  fanning-mill.  The  productivity  of  the 
plants  coming  from  the  various  classes  of  seed  was  compared 
under  field  conditions.  Some  investigators  procured  a  slightly 
greater  yield  from  plants  produced  by  heavy  seed  than  from  those 
coming  from  light  seed.  Others  obtained  no  such  difference. 
Plants  from  medium  or  ungraded  seed  in  almost  all  cases  proved 
as  productive  as  those  from  heavy  seed.  The  work  carried  on 
at  the  Ohio  Station  may  be  taken  as  a  typical  example  of,  these 
investigations. 

Table  XXIV  presents  the  average  results  (Williams  and  Welton, 
1911)  of  an  experiment  with  weight  of  seed  wheat  over  a  period  of 
seven  years.  The  grades  are  first,  second,  and  third,  represent- 
ing heavy,  medium,  and  light  seed,  respectively.  Two  methods 
of  seeding  were  practiced,  namely,  a  uniform  rate  by  weight  and 
a  varied  seeding  to  obtain  approximately  an  equal  number  of 
plants  on  equal  areas. 

TABLE  XXIV. — THE  RELATION  OF  WEIGHT  OF  GRAIN  TO  YIELD  IN  WHEAT 
Seven-year  Average  Results 


Grade 

Seed  used, 
Av.  wt.  per 
bu.,  Ib. 

Bushels  per  acre 

Crop 
Harvested 
av.  wt.  per 
bu.,  Ib. 

Uniform 
seeding 

Varied 
seeding 

Average  of 
both  series 

First  

61.6 

59.8 
57.7 

31.3 
31.4 
31.3 

31.3 
30.9 
30.7 

31.3 
31.2 
31.0 

59.4 
59.0 
59.0 

Second  

Third  

In  the  case  of  oats  (Williams  and  Welton,  1913)  a  greater 
difference  was  obtained  between  light  and  heavy  seed,  but  the  un- 
screened seed  yielded  only  a  little  less  than  the  large  seed.  Table 
XXV  presents  the  average  data  of  a  four-year  period. 


RESULTS  OF  SELECTION  WITH  SELF-FERTILIZED  CROPS     125 


TABLE  XXV. — THE  RELATION  OF  WEIGHT  OF  GRAIN  TO  YIELD  IN  OATS 
Four- Year  Average  Results 


Grade 

Seed  used 

Bushels  per  acre 

Crop 
harvested, 
av.  wt.  per 
bu.,  Ib. 

Av.  wt. 
per  bu.,  Ib. 

No.  per 
ounce 

Uniform 
seeding 

Varied 
seeding 

Average 
of  both 
series 

Light       .... 

27.5 
30.7 
27.3 

1,052 
1,684 
1,286 

59.0 
58.0 

58.4 

59.0 
55.3 

58.0 

59.0 
56.7 
58.2 

28.6 
28.4 

27.8 

Heavy 

Unscreened.  .  . 

A  current  popular  belief  is  that  plants  from  large  or  heavy  seeds 
yield  more  than  plants  from  light  or  small  seeds.  The  data  col- 
lected by  various  investigators  do  not  substantiate  this  view. 
As  a  matter  of  fact,  from  a  practical  viewpoint  it  would  be 
difficult  to  demonstrate  any  increase  in  yield  as  the  result  of  the 
use  of  a  fanning  mill.  The  fanning  mill,  however,  is  very  useful 
in  removing  weed  seeds  or  diseased  light  grains. 

SELECTION  FOR  THE  PURPOSE  OF  ISOLATING  PURE  LINES 

The  determination  of  the  better  selections  requires  at  least  five 
years.  Accordingly,  there  have  been  consistent  attempts  to 
find  some  character  or  characters  which  were  so  closely  associated 
with  yield  or  other  economic  qualities  that  they  were  of  actual 
selection  value.  If  such  could  be  found  it  would  be  possible  to 
use  them  as  checks  on  the  yield  results.  Manifestly  they  would 
be  of  especial  value  in  the  early  period  of  head  selection,  for  the 
results  from  short  rows  planted  from  individual  heads  are  not 
very  accurate  indications  by  which  to  discard  selections. 

In  this  connection  DeVries  (1907)  states  that  "  correlation 
between  botanical  marks  and  breeding  qualities  are  to  be  con- 
sidered as  reliable  guides  in  the  work  of  selection."  As  an  illus- 
tration of  such  correlations,  the  belief  that  there  is  an  association 
between  two-grained  spikelets  of  oats  and  yield  may  be  mentioned. 
Some  of  the  early  data  collected  at  Svalof  indicated  that  such 
was  the  case.  After  fifteen  years  further  study,  five  or  six  of 
the  best  yielding  oat  varieties  were  examined.  Some  were  three- 
grained  types  and  others  were  two-grained  types.  Newman  (1912) 
in  summarizing  these  results  concludes  that  "there  seems,  there- 
fore, to  be  no  definite  relationship  between  the  yield  of  a  given 
strain  and  the  number  of  kernels  per  spikelet  by  which  it  is  char- 
acterized." The  relationship  between  other  characters  was 


126 


BREEDING  CROP  PLANTS 


likewise  studied,  such  as  early  maturity  and  high  yield;  short- 
haired  rachilla  and  high  brewing  qualities  in  barley;  weight  of 
1,000  grains  in  wheat,  oats,  and  barley  and  yield;  stooling  with 
yield  and  quality;  size  of  spike  or  panicle  and  yield.  In  some 
cases  there  seemed  to  be  a  relation  between  yield  or  quality  and 
some  particular  character,  but  when  sufficient  numbers  were 
studied  no  consistent  association  between  any  one  morphological 
character  and  yield  was  found. 

Much  investigational  study  has  been  made  on  this  subject  by 
others  and  similar  conclusions  have  been  reached.  At  the  Minne- 
sota Station  correlations  between  yield  and  the  following  char- 
acters in  wheat  have  been  sought;  stooling,  height  of  plant,  size  of 
seed,  date  heading,  and  date  of  maturity.  In  some  seasons  the 
early  varieties  were  the  better  yielders  and  in  other  seasons  the 
later  varieties. 

Stooling  was  obtained  from  plots  in  which  plants  had  room  for 
individual  development,  and  the  correlation  of  stooling  and  yield 
was  computed  for  two  years  for  wheat,  oats,  and  barley.  Yield 
was  obtained  from  the  replicated  rod-row  test.  The  results 
showed  no  association  between  stooling  and  yielding  ability. 

Quite  consistent  association  between  weight  of  1,000  plump 
seed  and  yield  of  wheat  as  determined  by  the  rod-row  test 
was  obtained  as  is  here  shown. 

TABLE   XXVI. — CORRELATIONS  BETWEEN  WEIGHT  OF  1,000  PLUMP  SEED 
or  T.  vulgare  AND  YIELD 


Number  of  selections  or 
varieties  in  the  population 

Class  and  year 

Correlation  coefficient 

70 

Spring,   1914 

0.431+0.066 

70 

Spring,   1915 

0.519±0.059 

35 

Spring,   1917 

0.580  +  0.076 

63 

Spring,   1918 

0.109  +  0.084 

54 

Winter,  1916 

0.356  +  0.080 

83 

Winter,  1917 

0.436+0.060 

Fairly  consistent  results  of  this  nature  would  seem  to  show  that 
weight  of  seed  was  associated  with  high  yield  in  wheat.  Mont- 
gomery (1912)  isolated  more  than  a  thousand  pure  lines  of 
Turkey  winter  wheat  at  the  Nebraska  Station  and  found  both 
large-  and  small-seeded  strains  among  the  higher  yielders.  Simi- 
lar results  have  been  obtained  at  Svalof . 

A  study  of  the  correlation  between  lodging  and  morphological 


RESULTS  OF  SELECTION  WITH  SELF-FERTILIZED  CROPS     127 

characters  of  the  stems  of  cereals  has  been  carried  out  at  the 
Minnesota  Station  (Garber  and  Olson,  1919).  Number  of  fibro- 
vascular  bundles,  area  of  sclerenchyma  cells  in  the  cortex  and 
bundle  and  other  characters  were  studied  in  relation  to  lodging. 
Stiffness  and  thickness  of  wall  of  the  sclerenchyma  seemed  to  be 
associated  in  oats  but  no  such  relation  was  found  in  wheat  and 
barley.  No  other  instance  of  a  close  association  between  any 
one  of  the  characters  studied  and  lodging  was  obtained. 

Some  correlations  are  of  value  in  selection  or  in  obtaining 
accurate  data.  Thus,  if  one  desires  to  classify  a  number  of 
selections  according  to  comparative  maturity,  reliable  results 
may  often  be  obtained  by  taking  such  notes  as  date  of  awn  emer- 
gence in  barley  and  date  of  heading  in  wheat  and  oats.  In  years 
favorable  for  normal  development,  a  high  correlation  between 
date  of  heading  and  maturity  has  been  obtained.  In  unfavorable 
years,  date  of  heading  is  a  more  reliable  indication  of  the  inherited 
differences  between  strains  in  relation  to  their  normal  period 
of  maturity  than  a  note  taken  at  maturity. 

In  general,  it  seems  safe  to  conclude  that  no  one  character 
is  closely  enough  associated  with  yield  to  be  of  selection  value 
in  picking  out  the  highest  yielding  strain.  It  is  possible,  how- 
ever, in  many  crops  to  weed  out  the  very  undesirable  plants  by 
inspection.  The  yield  test  must  then  be  used  to  determine  the 
better  pure  lines.  This  seems  reasonable  when  we  realize  that 
yield  is  the  final  result  of  many  growth  characters.  A  strain  which 
excels  in  all  characters,  such  as  stooling,  disease  resistance,  size  of 
seed,  size  of  head,  fertility,  etc.,  naturally  will  be  a  high  yielder. 
As  so  many  characters — of  which  the  above  are  only  a  few  of  the 
more  easily  seen — are  essential  to  high  yield,  no  single  botanical 
character  is  of  great  selection  value.  This  has  led  to  the  present 
method  which  is  summarized  as  follows  by  Newman  (1912): 

"Thus  instead  of  basing  the  isolation  of  superior  individuals  purely 
upon  botanical  or  morphological  characters  as  was  formerly  the  case, 
the  principle  has  become  to  select  a  large  number  of  individuals  without 
special  regard  to  such  characters." 

The  value  of  these  individuals  is  determined  by  the  study 
of  yield  continued  over  several  years. 

Numerous  experiments  have  proved  the  value  of  this  method. 
In  this  connection  it  is  of  interest  to  point  out  progress  that  has 
already  been  made  with  self -fertilized  crops. 


128 


BREEDING  CROP  PLANTS 


WHEAT  SELECTIONS 

A  new  winter  wheat,  Kanred  (Jardine,  1917),  discovered  at 
the  Kansas  Experiment  Station  as  a  result  of  testing  out  554 
head  selections  made  from  Crimean  (No.  1,435  of  the  Office  of 
Cereal  Investigations,  United  States  Department  of  Agriculture) 
is  a  rather  striking  example  of  what  may  be  accomplished  by  this 
method  of  work.  As  an  average  of  six  years'  tests,  Kanred  yielded 
4.6  and  5.2  bu.  more  than  Turkey  and  Kharkov  respectively. 
These  varieties  gave  best  results  under  Kansas  conditions  until 
Kanred  was  found. 

Table  XXVII  shows  a  comparison  in  yield  between  commer- 
cial varieties  and  selections  made  from  them  (Love  and  Craig, 
1918a)  at  the  Cornell  Station. 

TABLE  XXVII. — THREE-YEAR  AVERAGE  YIELD  PER  ACRE  OF  WINTER  WHEAT 
VARIETIES  AND  SELECTIONS  MADE  FROM  THEM 


Varietal  selections 


Three-year     i 

average  yield  i      Gain,  bu. 
per  acre,  bu. 


Klondyke 

28  2 

Klondyke  126-26  

30.4 

2.2 

Klondyke  126-44  
Fulcaster 

31.3 
26  0 

3.1 

Fulcaster  123-23  

27.9 

1.9 

Fulcaster  123-32  (beardless)  

30.2 

4.2 

Red  Wave               

27  7 

Red  Wave  128-47.  . 

31.1 

3.4 

Red  Rock  winter  wheat,  which  is  highly  satisfactory  in  Michi- 
gan, comes  from  a  red  seed  picked  out  of  a  white  wheat  (Ply- 
mouth Rock)  (Spragg  and  Clark,  1916).  Here  we  have  an 
example  of  selecting  and  increasing  an  individual  obviously  differ- 
ent from  the  type  in  which  it  occurred.  The  red  seed  may  have 
been  due  to  one  of  several  causes,  admixture,  natural  crossing, 
or  a  mutation.  Whatever  the  cause,  selection  immediately 
isolated  a  wheat  which  was  different  in  appearance  and  which 
proved  valuable.  On  a  percentage  basis,  the  average  yield  of 
Plymouth  Rock  at  the  Michigan  Experiment  Station  during  the 
period  1912-1915  is  73.4.  The  yield  of  Red  Rock  for  the  same 
period  is  taken  as  100. 

Besides  yield  and  quality  other  characters  of  economic  impor- 


RESULTS  OF  SELECTION  WITH  SELF-FERTILIZED  CROPS     129 

tance  may  be  improved  by  selection.  The  illustration  below, 
taken  from  Williams  (1916),  shows  clearly  what  has  been  accom- 
plished at  the  Ohio  Station  in  the  way  of  isolating  a  strain  with 
stiff  straw.  The  three  pure  lines  shown  are  selections  from  the 
commercial  variety  Fultz. 


FIG.  26. — Variation  in  stiffness  of  straw  in  pure  line  selections  of  Fultz  wheat. 
(After  C.  G.  Williams.) 

OAT  SELECTIONS 

The  Maine  Experiment  Station  has  made  somewhat  extensive 
studies  of  pure  lines  in  oats  (Surface  and  Zinn,  1916).  The 
yields  of  commercial  varieties  were  compared  with  that  of  their 
respective  pure-line  selections.  In  Table  XXVIII  are  given  a 
part  of  the  data  reported  in  Maine  Agricultural  Bulletin  250. 

The  average  of  the  seven  pure  lines  of  Banner  for  the  entire 
period  of  the  test  is  81  bu.  per  acre,  while  that  of  the  commercial 
Banner  is  79.7  bu.  The  average  difference  for  the  period  of 
the  test  between  commercial  Irish  Victor  and  the  four  pure  lines 
is  nearly  6  bu.  per  acre. 

9 


130 


BREEDING  CROP  PLANTS 


TABLE    XXVIII. — YIELDS    OF   COMMERCIAL   VARIETIES   AND   PURE-LINE 

SELECTIONS 


Bushels 

per  acre 

Variety 

1913 

1914 

1915 

Mean 

Banner  (commercial)  

62  7 

94   5 

81  8 

79.7 

Banner,  Maine  355  (p.l.)  

71  0 

105  3 

83  6 

86  6 

Banner,  Maine  281  (p.l.)  
Banner,  Maine  351  (p.l.)  
Banner,  Maine  230  (p.l.)  
Banner,  Maine  307  (pi) 

73.1 
70.0 
69.4 
66  9 

97.0 
98.2 
93.8 
95  8 

81.2 
75.5 
76.8 
77  0 

83.8 
81.2 
80.0 
79  9 

Banner,  Maine  286  (p.l.)  

70.9 

87.1 

75.7 

77.9 

Banner,  Maine  357  (p.l.)  
Irish  Victor  (commercial)  
Irish  Victor,  Maine  340  (p.l.)  
Irish  Victor,  Maine  337  (p.l.)  
Irish  Victor,  Maine  336  (p.l.)  
Irish  Victor  Maine  346  (pi.)  

70.0 
67.0 
74.1 
58.4 
75.3 
71.9 

83.1 
82.4 
95.8 
103.9 
89.2 
89.5 

79.1 
76.2 
83.6 
79.2 
75.1 
77.0 

77.4 
75.2 
84.5 
80.5 
79.9 
79.5 

Kiesselbach  and  Ratcliff  (1917)  have  reported  in  Bulletin  160 
of  the  Nebraska  Experiment  Station  the  yields  of  numerous  pure 
lines  of  Kherson  together  with  the  yield  of  the  commercial  variety 
for  a  four-year  period. 


TABLE  XXIX. — YIELD  TEST  OF  KHERSON  OAT  STRAINS  GROWN  IN  FIELD 
PLATS.     1913  TO  1916 


Strain 


Yield  in  bushels  per  acre 


No. 

1913 

1914 

1915 

1916 

Average 

Original 

44.4 

58.9 

29.9 

-  _ 

83.0 

54.1 

21 

58.0 

71.7 

32.4 

85.4 

61.9 

23 

61.0 

67.3               27.7 

85.9 

60.5 

15 

51.8 

50.4                26.5 

77.3 

51.5 

25 

62.1 

64.2      . 

30.7 

83.9 

60.2 

6 

64.1 

63.2 

24.4 

81.6 

58.3 

33 

58.5 

63.7 

35.7 

86.1 

61.0 

27 

50.8 

67.1 

31.7 

81.9 

57.9 

38 

62.1 

.64.9 

31.9 

81.1 

60.0 

35 

33.9 

53.0 

22.8 

76.7 

46.6 

4 

61.0 

67.8 

30.5 

80.3 

59.9 

5 

65.2 

33.3 

83.3 

19 

50'.  1 

21.4 

74.8 

RESULTS  OF  SELECTION  WITH  SELF-FERTILIZED  CROPS      131 

As  shown  by  the  last  column  of  the  table,  only  two  of  the 
thirteen  pure  lines  gave  lower  average  yields  than  the  commer- 
cial variety  for  the  four-year  period.  They  are  Nos.  15  and  35. 

Among  oat  selections  (Anonymous,  1919)  which  have  proved 
their  practical  value  may  be  mentioned  Iowa  103,  Iowa  105,  and 
lowar,  all  of  which  are  pure-line  selections  from  Kherson.  These 
selections  were  made  by  Burnett  at  the  Iowa  experiment  station. 

SELECTIONS  IN  OTHER  SELF -FERTILIZED  CROPS 

An  exhaustive  account  of  the  work  that  has  been  done  in 
isolating  and  testing  pure  lines  of  self-fertilized  crops  would 
alone  make  a  large  volume.  In  this  somewhat  brief  treatment 
only  a  few  typical  examples  are  chosen. 

The  Iron  cowpea  (Orton,  1911),  which  is  resistant  to  wilt, 
is  one  of  the  notable  examples  of  what  has  been  accomplished 
by  the  introduction  of  a  promising  variety.  The  isolation  of 
this  form  alone  has  produced  thousands  of  dollars  for  the  farmer. 
M.  A.  C.  Robust  bean  (Spragg,  1919),  which  is  a  selection  out  of 
the  ordinary  navy  bean,  has  proved  to  be  very  much  superior 
in  yield  to  the  commercial  variety.  At  the  Svalof  Experiment 
Station  (Newman,  1912),  in  Sweden,  progress  has  been  made  in 
isolating  pure  lines  of  barley  which  possess  superior  brewing 
qualities. 

These  few  examples  show  the  value  of  selection  as  a  means 
of  crop  improvement.  The  effect  of  selection  is  to  isolate  the 
more  desirable  types  from  the  commercial  variety.  After  this 
has  been  accomplished,  crossing  may  be  resorted  to  as  a  method 
of  obtaining  a  variety  which  combines  the  desirable  characters  of 
several  strains. 


CHAPTER  X 

SOME  RESULTS  OF  CROSSING  AS  A  MEANS  OF 
IMPROVING  SELF-FERTILIZED  CROPS 

In  the  preceding  chapters  it  was  shown  that  the  selection 
and  increase  of  a  homozygous  individual  plant  isolated  a  pure 
line.  No  one  of  these  pure  lines  contains,  as  a  rule,  all  the 
characters  desired.  What  usually  happens  is  that  one  pure 
line  excels  in  one  character,  while  another  is  superior  with 
regard  to  some  other  character.  The  only  way  in  which  the 
desirable  characters  belonging  to  different  strains  can  be  com- 
bined is  by  crossing  and  then  selecting  the  desired  segregate. 

To  attain  success  in  this  field,  it  is  important  to  use  as  parents 
those  forms  which  most  nearly  approach  the  combination  of 
characters  desired.  The  old  idea  of  indiscriminate  crossiiig  in 
order  to  procure  superior  economic  characters,  such  as  yield, 
has  been  largely  abandoned,  which  is  reasonable  from  our  knowl- 
edge of  what  selection  accomplishes  and  of  Mendel's  law  of  in- 
heritance in  crosses.  Love  (1914)  compared  the  yield  of  oat 
selections  with  hybrids  which  were  the  result  of  more  or  less 
indiscriminate  crosses  made  by  J.  B.  Norton.  The  average 
yield  of  the  hybrids  was  but  little  higher  than  the  average  yield 
of  the  selections.  It  is  probable  that  the  comparison  would  have 
shown  a  greater  difference  if  the  parents  had  been  chosen  on  the 
basis  of  their  performance  records.  Two  forms  may  be  crossed 
because  each  possesses  to  the  greatest  degree  the  character  sought, 
with  the  hope  of  obtaining  transgressive  segregation;  or  a  cross 
may  be  made  to  combine  different  characters. 

The  Improvement  of  Black  Oats  at  Svalof.— Nilsson-Ehle 
(1917)  has  reported  experiments  carried  on  from  1901  to  1917  for 
the  purpose  of  improving  the  black  oats  grown  in  Sweden.  The 
native  oats  formerly  grown  had  weak  straw  and  lodged  badly. 
Black  Tartarian  oats  was  introduced  to  overcome  this  difficulty. 
Little  by  little  this  form  mixed  with  the  native  oats  and  probably 
naturally  crossed  to  some  extent.  The  resultant  complex  (Svart 
Tartarish  Plymhafre)  was  especially  suitable  for  selection  and  the 

132 


IMPROVING  SELF-FERTILIZING  CROPS 


133 


isolation  of  desirable  forms.  This  was  done  at  the  Svalof  Station. 
The  selections,  Klock  1  and  Stormogul,  maturing  early  and  late 
respectively,  were  obtained.  Both  possess  stiff  straw  and  Storm- 
ogul has  good  yielding  ability.  The  improvement  of  the  latter 
character  was  sought  by  crossing  with  higher  yielding  light 
colored  forms.  The  following  diagram  indicates  the  method 
followed.  The  varieties  and  strains  were  purified  before  the 
crosses  were  made. 


Svart  Tartarish  Plymhafre 


Milton  (Probsteier  type) 


I 
Klock  I  (1901)  X  Guldregn  (1903) 

I 
Stormogul  (1901)  X  Klock  II  (1909) 

Klock  III  (1917) 

Klock  II  is  the  result  of  crossing  a  good-yielding  black  oat  of 
stiff  straw  (Klock  I)  with  a  high-yielding  yellow  oat  (Guldregn). 
The  offspring  has  the  stiff  straw  of  Klock  I  and  the  high-yielding 
ability  of  Guldregn.  One  selection  in  the  cross  of  Klock  II  with 
Stormogul  gave  a  strain,  Klock  III,  which  has  the  early  maturity 
of  Klock  II,  a  somewhat  higher  yielding  ability  than  Stormogul, 
as  well  as  non-lodging  ability,  which  last  character  both  parents 
possessed.  In  Table  XXX  the  yields  of  three  of  the  strains 
are  shown. 

TABLE  XXX. — RESULTS  OF  COMPARATIVE  YIELD  TRIALS  OP  THE  VARIETIES 

KLOCK  II,  STORMOGUL,  AND  A  SEGREGATE  KLOCK  III  OF  A  CROSS 

BETWEEN   STORMOGUL    X    KLOCK   II   AS   OBTAINED   AT   SVALOF 

FROM  1912  TO  1916 


Yield  pe 

r  hectare 

Relative 

Grain 

1912, 
kg. 

1913, 
kg. 

1914, 
kg. 

1915, 
kg. 

1916, 
kg. 

Average, 
kg. 

Klock 
II  =  100 

Klock  III  
Stormogul  
Klock  II  .  . 

3,780 
3,860 
3  730 

4,170 
4,160 
3  870 

2,560 
2,700 
2360 

3,010 
3,030 
2  280 

4,580 
4,160 
4  230 

3,620 
3,582 
3  284 

109.9 
108.7 
100  0 

Straw 

Klock  III  

5,060 

4530 

2  470 

3  825 

7  850 

4  747 

100  3 

Stormogul  .... 
Klock  II 

5,810 
5  260 

5,330 
4  470 

2,850 
2  310 

4,550 
4  300 

7,630 
7  330 

5,234 
4  734 

110.6 
100  0 

134  BREEDING  CROP  PLANTS 

A  Wheat  Cross  Made  at  Svalof.— The  highest  yielding  winter 
wheat  grown  at  the  Svalof  Station,  reported  by  Newman  (1912), 
was  a  cross,  Extra  Squarehead  II,  No.  0290.  This  wheat  is 
one  of  the  offspring  of  Old  Extra  Squarehead  X  Grenadier  II. 
It  combines  the  winter-hardiness  and  rust  resistance  of  the  former 
with  the  stiff  straw  and  high  yield  of  the  latter.  As  an  average 
of  four  years'  trial  at  Svalof  and  Alnarp,  1his  wheat  has  yielded 
18  per  cent,  more  than  Old  Extra  Squarehead  and  8  per  cent, 
more  than  Grenadier  II,  which  was  next.  No  variety  of  winter 
wheat  has  proved  so  generally  popular  among  the  farmers  of 
southern  Sweden  as  Extra  Squarehead  II.  It  may  be  of  interest 
to  point  out  that  preceding  the  cross,  hundreds  of  selections  out 
of  Grenadier  II  were  examined  in  search  of  a  pure  line  with  the 
combination  of  rust  resistance  and  high  yield. 

Wheat  Breeding  at  University  Farm,  Cambridge,  England. — 
Most  of  the  wheat  varieties  grown  in  England  are  very  susceptible 
to  yellow  rust  (Puccinia  glumarum).  Biffen  (1917)  set  himself 
the  task  of  breeding  a  high-yielding,  resistant  form.  He  crossed 
American  Club,  which  is  very  resistant  to  this  parasite,  with 
several  susceptible  varieties  in  order  to  study  the  mode  of  in- 
heritance and  develop  a  standard  technic  of  operations.  In  all 
crosses  the  Fz  generations  showed  monohybrid  segregation  with 
resistance  behaving  as  the  recessive.  The  resistant  individuals 
were  rather  clear-cut,  although  they  sometimes  exhibited  uredinia. 
The  susceptible  plants  showed  a  wide  range  of  variation.  No 
recognizable  morphological  character  has  been  found  correlated 
with  resistance. 

The  constancy  of  resistance  in  wheats  of  hybrid  origin  has  also 
been  studied  by  Biffen.  For  the  purpose  he  used  a  resistant 
strain  produced  from  a  cross  between  American  Club,  a  resistant 
variety,  and  Michigan  Bronze,  which  is  one  of  the  forms  most 
susceptible  to -yellow  rust.  During  eight  years  of  observation 
the  hybrid  variety  proved  just  as  resistant  as  the  American  Club. 

A  resistant  variety  of  Russian  origin,  found  among  some 
Gurka  wheats,  which  was  not  adapted  to  local  conditions,  was 
crossed  with  Square  Head's  Master,  the  variety  most  commonly 
grown  in  England.  Among  the  resistant  offspring  is  one  that 
gives  considerable  promise.  Comparative  trials  of  this  wheat 
(Little  Joss)  over  a  period  of  seven  years  show  it  to  yield  about  4 
bu.  per  acre  more  than  the  best  of  the  English  and  French  wheats. 
The  explanation  for  this  would  seem  to  be  that  Little  Joss  in- 


IMPROVING  SELF-FERTILIZING  CROPS  135 

herited  the  yielding  capacity  of  Square  Head's  Master  as  well  as 
the  resistance  of  the  Russian  wheat  parent. 

Farrer's  Wheat  Breeding  in  Australia. — Probably  no  one  has 
made  more  wheat  crosses  that  have  proved  valuable  than  William 
Farrer  of  Australia  (Sutton,  1910).  Most  of  his  work  was  done 
without  the  application  of  a  knowledge  of  the  Mendelian  princi- 
ples. He,  however,  made  crosses  for  definite  purposes  and  in 
reality  followed  the  Mendelian  mode  of  work  without  recognizing 
the  law  involved.  Farrer  strongly  featured  composite  crossing, 
i.e.,  the  crossing  of  parents  which  were  themselves  of  hybrid 
origin.  Federation,  a  variety  very  popular  in  southern  Australia, 
was  produced  in  this  manner.  As  a  typical  example  of  Farrer's 
method,  the  history  of  Federation  will  be  given  somewhat  in 
detail. 

This  variety  was  the  outcome  of  a  deliberate  attempt  to 
produce  a  wheat  especially  suited  to  gathering  with  a  stripper, 
a  harvester  used  in  Australia.  Federation  is  early  maturing, 
stiff-st rawed,  erect,  and  of  somewhat  short  growth.  Despite 
its  rather  unattractive  appearance,  it  is  one  of  the  highest 
yielding  wheats  for  the  section  in  which  it  is  grown.  The  upright 
habit  makes  it  easy  to  harvest.  Furthermore,  the  grains  are 
held  tight  enough  to  prevent  shattering  but  not  tight  enough 
to  interfere  with  the  operation  of  the  stripper.  Federation 
resulted  from  a  cross  between  the  varieties  Purple  Straw  and  Yan- 
dilla.  The  parentage  is  indicated  in  the  following  diagrammatic 
scheme: 

Improved  Fife  X  Etawah 
Purple  Straw  X  Yandilla 
Federation 

The  history  of  the  origin  of  Bunyip,  another  Farrer  production, 
is  indicated  as  follows: 

Improved  Fife  X  Purple  Straw        Blount's  Lambrigg  X  Hornblende 

I 
An  unnamed 

cross-bred  X  King's  Jubilee 
Rymer  X  Maffra 

Bunyip 


136  BREEDING  CROP  PLANTS 

Among  other  varieties  produced  by  crossing  which  are  of 
economic  importance  may  be  mentioned  Comback,  Cedar, 
Firbank,  Bobs,  Florence  and  Cleveland. 

Farrer's  method  of  breeding  seems  to  have  been  based  on 
inducing  maximum  variation  through  composite  crossing  and  then 
subjecting  the  progeny  to  selection.  He  was  a  keen  observer  and 
possessed  ability  to  pick  out  forms  which  proved  of  economic 
value.  This  emphasizes  the  need  of  a  knowledge  of  the  charac- 
ters of  a  crop  with  which  the  breeder  is  to  work,  which  is  as 
essential  as  a  knowledge  of  laws  of  breeding. 


FIG.  27. — A  section  of  the  winter  wheat  plant  breeding  nursery  in  the  spring 
of  1918.  The  three  rows  at  the  right  are  Minhardi,  a  very  winter-hardy  wheat 
produced  from  a  cross  of  Odessa  with  Turkey.  In  right  center  are  three  rows  of 
Turkey,  Minn.  1487. 

Marquis  Wheat. — If  the  spring  wheat  known  as  Marquis 
(Saunders,  1912)  were  the  only  one  of  economic  importance  which 
had  been  produced  by  artificial  crossing,  the  practice  would  be 
justified.  The  early  history  of  this  wheat  is  somewhat  obscure. 
It  is  one  of  the  descendants  of  a  cross  between  an  early  ripening 
wheat  from  India,  Hard  Red  Calcutta  9  and  Red  Fife  cf .  The 
cross  was  made  by  A.  P.  Saunders,  probably  at  the  experimental 
farm  at  Agassiz,  Canada,  in  1892.  The  crossed  seed  or  its 
progeny  was  transferred  to  the  Ottawa  Experimental  Farm. 
In  1903  Chas.  E.  Saunders  took  charge  of  the  cereal  breeding  at 
this  place  and  immediately  initiated  a  series  of  selections  from  the 


IMPROVING  SELF-FERTILIZING  CROPS  137 

progeny  of  cross-bred  wheats.  The  progeny  from  the  cross 
made  by  A.  P.  Saunders  was  found  to  differ  strikingly  in  gluten 
content  of  seeds.  The  laborious  practice  of  chewing  a  small 
sample  of  each  pure  line  was  made  the  basis  of  selections .  One 
of  the  high-gluten  selections  isolated  from  this  mixture  of  types 
was  named  Marquis.  It  was  first  grown  as  a  pure  form  in  1904 
and  the  bread-making  tests  made  in  1907  fully  established  its 
bread-making  qualities.  In  addition  to  this  character  it  is  early 
ripening,  thus  often  escaping  rust,  has  stiff  straw,  high  yielding 
ability,  distinctive  appearance  of  seed,  and  remarkably  wide 
adaptations.  These  qualities  have  made  it  popular  among  the 
farmers  in  the  spring  wheat  belt. 


FIG.    28. — Minhardi,    Minn.    1505.     Grown    in    1918.     This    variety    is    very 

winter-hardy. 

Winter  Wheat  Breeding  at  the  Minnesota  Agricultural  Experi- 
ment Station. — One  of  the  most  urgent  needs  in  order  to  bring 
about  the  successful  production  of  winter  wheat  in  Minnesota 
is  a  strain  which  will  withstand  the  severe  winters.  This  ideal 
has  more  or  less  been  the  goal  of  breeding  operations  from  the 
first.  Yield  and  quality  also  have  been  given  considerable 
attention.  Before  attempting  crossing,  varieties  were  obtained 
from  all  over  the  world.  Odessa,  an  awnless,  red-chaffed 
variety  of  Russian  origin,  has  proved  most  winter-hardy ,  although 


138 


BREEDING  CROP  PLANTS 


some  of  the  more  recent  Turkey  selections  are  nearly  as  hardy. 
In  the  large  number  of  crosses  that  have  been  studied  since  1902 
there  is  an  outstanding  fact  worthy  of  emphasis  from  a  plant 
breeding  viewpoint.  Of  the  different  crosses  made,  none  proved 
as  winter-hardy  as  the  Odessa-Turkey  combination,  although 
numerous  crosses  between  other  winter  wheats  were  studied. 
This  shows  the  necessity  of  studying  carefully  prospective  paren- 
tal material  to  determine  what  should  be  used.  When  Odessa 
was  used  it  furnished  an  hereditary  complex  capable  of  with- 
standing severe  winters  (Hayes  and  Garber,  1919). 


FIG.  29.— Turkey,    Minn.    529.     Grown   in    1918. 

very  badly. 


This   variety   winter-killed 


At  University  Farm  and  at  Waseca  one  of  the  Odessa-Turkey 
crosses,  Minhardi,  (Minnesota  No.  1,505)  has  proved  more 
winter-hardy  than  the  Odessa  parent.  This  cross  also  possesses 
very  high  yielding  ability  but  the  quality  of  seed  is  somewhat 
inferior.  Its  ability  to  yield  is  probably  inherited  from  the 
Turkey,  which  yielded  high  in  favorable  seasons.  Minturki 
(Minnesota  No.  1,507)  is  a  bearded  wheat  obtained  from  a  cross 
of  Odessa  with  Turkey.  It  is  somewhat  less  winter-hardy  than 
Minhardi  but  it  excels  in  quality  and  yielding  ability.  Table 
XXXI  presents  data  on  some  of  the  more  promising  forms  of 
winter  wheat  for  Minnesota  conditions. 


IMPROVING  SELF-FERTILIZING  CROPS 


139 


TABLE  XXXI. — AVERAGE  YIELDS  AND  AVERAGE  WINTER  INJURY  OF  THE 

BETTER  WINTER  WHEATS  GROWN  AT  UNIVERSITY  FARM  AND  AT  THE 

WASECA  SUBSTATION 


Variety  or 

N.S.N. 

Minne- 
sota 
acces- 

Minne- 
sota 

University  farm 

Waseca, 
1918 

Average 
winter 

cross 

sion 
No. 

No. 

1916, 
bu. 

1917, 
bu. 

1918, 
bu. 

bu. 

injury, 
per  cent. 

Turkey  

1-03-213 

829 

829 

24.7 

30.0 

7.7 

40.1 

51 

Odessa  

1-01-3 

558 

1,471 

32.9 

36.1 

24.8 

30.6 

38 

Turkey  X  Odessa 

11-02-195 

829  X  558 

1,505 

37.0 

43.1 

40.9 

35.3 

29 

Turkey  X  Odessa 

11-02-280 

829  X  642 

1,507 

38.7 

47.5 

20.9 

32.5 

42 

Turkey  

1-03-68 

529 

1,487 

27.5 

39.0 

5.3 

20.2 

73 

Turkey 

1-03-120 

1,488 

1,488 

33.4 

40.9 

36.5 

36.4 

40 

The  pure-line  parentage  of  the  Turkey-Odessa  crosses  is  not 
known,  although  the  parents  are  believed  in  all  cases  to  have 
originated  from  a  single  plant.  At  the  time  the  crosses  were  made, 
in  1902,  there  was  no  pedigreed  Turkey  available  with  the  winter- 
hardy  ability  of  Minnesota  1488.  One  of  the  recent  crosses  made 
is  between  the  best  Turkey  selection,  Minnesota  1488,  and  Min- 
hardi.  Both  parents  are  winter-hardy  and  are  good  yielders;  the 
Turkey  likewise  produces  good  quality  seed.  This  shows  what  is 
believed  to  be  the  correct  procedure  in  plant  breeding. 

Breeding  Beans  Resistant  to  Colletotrichum  Lindemuthia- 
num. — Extensive  tests  of  the  reaction  between  physiological 
strains  of  anthracnose  and  host  plants  were  made  at  the  Cornell 
Station.  Four  groups  of  beans  were  obtained;  (1)  Resistant  to 
both  strains,  (2)  resistant  to  strain  A  and  susceptible  to  strain  F, 
(3)  susceptible  to  A  and  resistant  to  F,  (4)  susceptible  to  both  F 
and  A.  Wells'  Red  Kidney  was  practically  immune  to  strain  A 
and  highly  resistant  to  strain  F,  while  Michigan  Robust  carried 
resistance  to  the  F  strain  only.  The  latter  is  a  white  navy  bean 
of  superior  yielding  ability  as  was  pointed  out  in  the  preceding 
chapter.  McRostie  (1919)  crossed  these  two  varieties  to  obtain 
a  bean  which  possessed  in  addition  to  the  characteristics  of 
Robust,  resistance  to  strain  A  of  anthracnose.  Segregation 
occurred  for  resistance  to  strain  A  on  a  simple  Mendelian  3: 
1  ratio  with  susceptibility  recessive.  In  the  second  and  third 
generations,  a  white  navy  bean  homozygous  for  resistance  to 
both  physiological  strains  of  anthracnose  was  obtained. 

An  Improved  Strain  of  Tobacco. — Connecticut  Havana  tobacco 
introduced  among  Wisconsin  farmers  gave  satisfactory  results  as 


140 


BREEDING  CROP  PLANTS 


to  quality  but  the  yield  was  low.  Johnson  (1919),  of  the  Wiscon- 
sin Agricultural  Experiment  Station,  attempted  to  overcome  this 
objection  by  breeding.  In  1909  a  pure-line  study  revealed  the 
fact  that  there  were  no  less  than  three  distinct  morphological 
types  present  in  the  particular  variety,  grown  at  the  experiment 


FIG.  30. — Tobacco  No.  27.     A  pure  line  strain  with  a  high  leaf  number  and  a 
low  breadth  index  of  leaf.     (After  Johnson,  1919.) 


station,  which  was  introduced  from  Connecticut.  Selections 
No.  26  and  "No.  27  differed  distinctly  from  the  normal  or  prevailing 
type.  Form  26  carried  fewer  leaves  but  of  larger  size  than  the 
normal,  while  form  27  possessed  more  leaves  which  were  some- 
what smaller  in  size  than  the  normal.  A  cross  between  269 
and  27  cf  was  made  in  1910  with  the  hope  of  combining  the 
desirable  features  of  the  two  forms.  The  success  of  the  cross  is 
indicated  in  the  following  data  taken  from  Johnson. 


IMPROVING  SELF-FERTILIZING  CROPS 


141 


TABLE  XXXII. — SUMMARIZED  DATA  OP  MOST  SIGNIFICANT  CHARACTERS 

OP  CONNECTICUT  HAVANA  No.  38  TOGETHER  WITH  PARENT  AND 

NORMAL  STRAINS.     AVERAGE  OP  EIGHT  YEARS 


Strain 

• 

Leaf  No. 

Average  of  to] 
bottom 

3,  middle,  and 
leaves 

Breadth 
index  of 

Length,  in. 

Width,  in. 

leaf 

No  26 

14   2 

20  0 

11.3 

56  5 

No.  27  

18.0 

18.0 

9.6 

53.6 

No.  38.      =  26  X  27 
No.  33  

16.9 
15.5 

19.1 
18.2 

10.6 
9.8 

55.8 
53.8 

FIG.  31. — Tobacco  No.  26.  A  pure  line  strain  with  a  low  leaf  number  and  a 
high  breadth  index  of  leaf.  Note  the  method  of  insuring  self-fertilization  by 
covering  the  terminal  inflorescence  with  a  manila  paper  bag.  (After  Johnson, 
1919.) 

No.  33  is  a  desirable  strain  of  the  normal  Connecticut  Havana 
type  produced  by  continued  selection  and  inbreeding.  Breadth 


142 


BREEDING  CROP  PLANTS 


index  is  obtained  by  dividing  the  average  leaf  breadth  by  the 
average  length  and  multiplying  by  100.  The  table  shows  that 
by  crossing,  a  form,  No.  38,  was  obtained  which  combined  some- 
what the  desirable  features  of  the  parents  (Nos.  26  and  27) 
and  is  superior  in  both  number  and  size  of  leaves  to  the  better 
pure-line  obtained  by  selection  (No.  33).  As  an  indication  of 

the  commercial  reception  of 
this  new  form,  it  was  esti- 
mated that  at  least  10,000 
acres  of  No.  38  were  grown 
in  Wisconsin  in  1919  cut  of 
a  total  of  about  40,000 
acres.  Here  we  have  an 
example  of  crossing  two 
closely-related  forms  and 
obtaining  from  the  resul- 
tant progeny  a  strain  of 
more  commercial  value 
than  either  parent. 

The  illustrations  bring 
out  more  clearly  some  of 
the  features  of  the  parents 
and  progeny. 

Summary.  —  In  this 
chapter  concrete  evidence 

FIG.   32.-Tobacco    No.    38.     This   strain  °f  the  Value  °f   crossing  as 

was  produced  by  crossing  No.   26,  which  ex-  a  means    of    producing  im- 

cels  in  leaf   breadth,    with    No.  27,  which  is  nrftvpj      VJ,riWiV<a     nf     <?plf 

homozygous  for  high  leaf  number.     This  new  P^Ved     varieties     Ol      SeJt 

strain  is  more  desirable  than  any  other  pure  fertilized     crops     has    been 

line  form   obtained.     It  is  widely  grown  in  „  ,    j        /-,  u      1.4 

Wisconsin.    (After  Johnson,  1919.)  presented.     Crosses  should 

be   made    with   a   definite 

purpose  in  view  and  the  parents  should  be  selected  on  the  basis  of 
performance  records.  Just  as  a  chemist  requires  a  certain  knowl- 
edge of  the  elements  which  he  synthesizes  into  compounds,  so 
also  the  plant  breeder  may  make  crosses  much  more  intelligently 
if  he  is  thoroughly  acquainted  with  the  prospective  parental 
material.  Promiscuous  crossing  as  a  means  of  producing  im- 
proved forms  is  discouraged. 


CHAPTER  XI 
COWPEAS,  SOYBEANS,  AND  VELVET  BEANS 

Cowpeas,  soybeans,  and  velvet  beans  belong  to  the  group 
of  naturally  self-fertilized  crops.  The  fundamental  principles 
involved  in  breeding  crops  of  this  group  have  already  been  dis- 
cussed. It  suffices  here  to  point  out  that  the  method  of  breeding 
these  three  legumes  does  not  differ  essentially  from  that  for  the 
group. 

COWPEAS  (Vigna  sinensis) 

Origin. — A  wild  plant  closely  related  to  the  cultivated  cowpea 
grows  quite  generally  over  the  continent  of  Africa.  The  wild 
form  differs  from  the  cultivated  in  having  smaller  seeds  and  in 
having  pod  valves  which  coil  in  ripening.  The  two  forms  may 
be  hybridized  with  ease.  This  fact  and  the  fact  that  wild  cow- 
peas  have  been  found  in  no  other  place,  are  generally  accepted  as 
evidence  (Piper,  1916)  that  the  cultivated  form  arose  in  Africa. 

Description  and  Inheritance. — The  cowpea  resembles  the 
garden  bean  in  general  appearance.  Some  varieties  grow  erect 
while  others  are  vine-like  and  trail  over  the  ground.  The  pods 
are  rather  long  and  contain  from  6  to  15  seeds  each.  Flowers 
are  white  or  nearly  white  and  pale  to  medium  violet  purple  and 
are  shaped  like  those  of  the  garden  pea.  Seed  coats  vary  a  great 
deal  in  color — some  are  mottled,  others  uni-colored.  The  life 
period  of  this  plant  is  too  long  to  permit  its  growth  very  far 
north,  and  for  this  reason  an  earlier  maturing  cowpea  is  desirable. 

Size  and  shape  of  pod  and  seed  have  been  used  to  separate  the 
larger  groups.  No  studies  of  inheritance  of  these  major  differ- 
ential characters  have  been  made. 

Color  inheritance  with  particular  reference  to  the  seed-coat 
has  been  studied  by  Spillman  (1911)  and  more  recently  by 
Harland  (1919a,  6,  c,  1920).  Anthocyanin  coloration  in  the  stem 
and  leaf  stalk  is  dependent  on  a  single  factor  difference  X, 
dominant  to  its  absence.  The  inheritance  of  seed  coat  pattern 
involves  factors  B  (black),  N  (buff),  M  (Maroon)  and  R  (Red), 

143 


144  BREEDING  CROP  PLANTS 

Factor  system  for  seed  coat  colors: 

Black B  N  M  R 

Black B  N   m  R 

Black B  n    m  R 

Black B  n    M  R 

Brown b  N  M  R 

Buff b  N   m  R 

Maroon b  n    M  R 

Red 6  n    m  R 

New-Era  pattern E  R 

White Absence  of  R 

Purple  color  of  the  ripe  pod  is  dependent  on  one  main  factor 
difference  P.  Each  of  the  three  factors  B,  E,  and  P,  produces 
anthocyanin  pigmentation  in  the  young  pod,  calyx,  and  peduncle. 
Whether  these  three  factors,  each  dominant  to  its  absence,  con- 
stitute a  triple  series  of  multiple  allelomorphs  or  occupy  different 
loci  very  near  together  in  the  same  chromosome,  has  not  yet 
been  established. 

In  crosses  between  black  cowpeas  and  the  variety  Black  Eye, 
Spillman  found  the  patterns  known  as  Holstein  (pigmented  area 
covering  micropylar  end  and  isolated  spots  of  pigment  on  the  non- 
pigmented  area)  and  Watson  Eye  (pigmented  area  around  hilum 
with  indistinct  margin  at  micropylar  end  of  seed ;  micropylar  end 
covered  with  fine  dots  of  pigment)  appearing  in  the  Fz  generation. 
This  indicated  the  origin  of  varieties  which  bear  these  seed-coat 
patterns. 

The  inheritance  of  flower  color  in  the  cowpea,  according 
to  Harland,  is  rather  simple.  In  crosses  between  dark  and  pale, 
also  between  dark  and  white,  the  segregation  in  the  F2  generation 
proved  to  be  that  of  a  monohybrid  with  dark  behaving  as  the 
dominant.  Spillman  (1913)  found  correlations  between  the 
production  of  certain  seed-coat  colors  and  the  occurrence  of 
anthocyan  in  the  flowers. 

Root-knot  (Heterodera  radicicola)  and  wilt  (Neocosmospora 
vasinfecta,  var.  tracheiphila)  are  the  two  most  serious  diseases  of 
cowpeas.  The  former  is  due  to  the  attack  of  a  nematode  whereas 
the  latter  is  due  to  a  fungus.  The  variety  known  as  the  Iron  cow- 
pea  possesses  resistance  to  both  of  these  diseases.  According  to 
Orton  (1911)  this  disease  resistance  is  inherited  as  a  dominant 
character.  The  F2  generation  is  too  variable  to  be  satisfactorily 
explained  on  a  monohybrid  basis,  However,  it  behaves  in  a 


COWPEAS,  SOYBEANS,  AND  VELVET  BEANS  145 

Mendelian  way  and  hence  is  relatively  easy  to  transfer  and  iso- 
late by  crossing  and  selection. 

Some  Results  of  Selection  and  Crossing. — The  characteristics 
of  an  ideal  cowpea  are  resistance  to  nematodes  and  wilt,  upright 
habit  of  growth  with  pods  borne  high,  and  high  yielding  ability. 
With  this  ideal  in  view  the  United  States  Department  of  Agri- 
culture has  conducted  extensive  investigations. 

Attention  was  first  called  to  the  Iron  cowpea  by  T.  S.  Williams 
of  Monetta,  S.  C.  He  found  it  would  thrive  on  "pea-sick"  soil 
where  other  varieties  were  a  complete  failure.  On  learning  of 
this  resistant  variety,  Orton  gave  it  a  thorough  trial  and  found 
it  possessed  resistance.  Measures  were  immediately  taken  to  in- 


FIG.  33. — Iron  cowpea  vs.  Black  and  Taylor,  showing  comparative  resistance 
to  the  wilt  and  the  root-knot.  Iron  in  center  and  Black  and  Taylor  at  right 
and  left  respectively.  (After  Orton.) 

crease  and  disseminate  the  Iron  cowpea  generally  throughout  the 
southern  United  States.  Because  the  Iron  variety  did  not  pro- 
duce as  large  yields  of  seed  and  forage  as  some  other  varieties 
such  as  Unknown,  breeding  was  resorted  to  for  the  purpose  of 
producing  a  high-yielding  resistant  strain  (Webber  and  Orton, 
1902;  Orton,  1902). 

In  addition  to  disease  resistance  this  variety  has  a  relatively 
upright,  bushy  habit  of  growth  but  the  seed  production  is  low. 
At  first  a  large  number  of  sprawly  forms,  such  as  Red  Ripper, 
Clay,  Black,  and  Unknown  were  crossed  with  Iron.  None  of 
the  segregates  from  these  crosses  proved  particularly  desirable. 
10 


146  BREEDING  CROP  PLANTS 

Later  more  attention  was  given  to  the  selection  of  parents  on  the 
basis  of  habit  of  growth,  fruitfulness,  and  position  of  pods.  The 
necessity  of  a  selection  of  parents  on  the  basis  of  desired  char- 
acters can  not  be  over-emphasized.  Whippoorwill  and  New  Era 
are  desirable  varieties  with  respect  to  the  three  characters  men- 
tioned above.1  The  variety  Monetta  was  the  best  segregate 
obtained  by  Orton  from  a  cross  between  Whippoorwill  and  Iron. 
Brabham,  a  variety  which  has  consistently  shown  itself  superior 
to  Monetta,  is  the  result  of  the  same  cross  made  by  a  farmer. 
Both  of  these  varieties  of  hybrid  origin  possess  disease  resistance 
and  to  a  certain  degree  the  other  desirable  agronomic  characters. 
More  recently  Morse,  of  the  Forage  Crop  Investigations  Office, 
Bureau  of  Plant  Industry,  has  crossed  Brabham  with  Groit  (a 
hybrid  of  Whippoorwill  and  New  Era).  Victor,  one  of  the  segre- 
gates of  this  cross,  will  be  distributed  in  the  near  future.  Con- 
cerning the  merits  of  this  new  variety,  Piper  makes  the  following 
statement : 

"Victor  cowpea  is  absolutely  resistant  to  nematodes  and  wilt,  is  a  tall 
bushy  variety,  extremely  fruitful,  and,  all  in  all,  it  seems  conservative 
^o  say  it  is  by  far  the  best  variety  of  cowpea  ever  yet  developed." 

SOYBEANS  (Soja  max) 

Origin. — The  soybean  is  of  ancient  cultivation.  Japan,  China, 
Korea,  Manchuria,  northern  India,  and  the  Islands  of  Java 
have  grown  this  plant  for  centuries  both  as  a  human  food  and  as 
feed  for  animals.  In  Japan  and  Manchuria  the  cultivated  soy- 
bean is  erect  in  growth.  Its  nearest  wild  relative  is  a  small- 
stemmed,  trailing  plant  with  smaller  flowers,  pods  and  seeds. 
This  wild  form  is  found  in  Japan,  Manchuria,  and  China.  The 
varieties  of  soybeans  found  in  India  are  intermediate  between  the 
two  types  just  mentioned.  According  to  Piper  and  Morse  (1910) 
all  inter  grades  between  the  wild  plant  and  the  cultivated  erect 
form  may  be  found,  so  there  is  little  doubt  that  all  forms  belong 
to  one  species  (Soja  max). 

Classification  and  Inheritance. — The  numerous  varieties  of 
soybeans  show  many  different  combinations  of  characters. 
Varieties  differ  in  habit  of  growth,  some  being  erect,  others  more 
procumbent  and  several  truly  vining.  Color  and  shape  of  seed 
and  pods,  color  of  flowers,  color  of  pubescence  of  the  pod  and 

1  The  following  information  was  furnished  by  the  courtesy  of  DR.  C.  V. 
PIPER. 


COWPEAS,  SOYBEANS,  AND  VELVET  BEANS 


147 


time  of  maturity  are  characters  which  have  been  widely  used  in 
varietal  and  group  classifications. 

Little  work  has  been  done  on  the  inheritance  of  characters  in 
soybeans.  Beans  with  green  cotyledons  may  have  green  seed- 
coats,  while  beans  with  yellow  cotyledons  may  have  either  green 
or  yellow  seed-coats.1  H.  Terao  (1918)  of  the  Imperial  Agricul- 
tural Experiment  Station,  Tokyo,  Japan,  has  discovered  that 
in  a  cross  of  green  cotyledons,  green  seed-coats  9  X  yellow 
cotyledons,  yellow  seed-coats  d71  —  the  inheritance  of  the  green 
seed-coat  apparently  was  matroclinal;  likewise  the  inheritance  of 
the  character  of  the  cotyledons.  In  the  reciprocal  cross  the 
character  of  the  cotyledons  again  proved  matroclinal  in  inherit- 
ance but  the  seed-coat  character  segregated  as  a  monohybrid 
with  green  dominant.  In  explanation  of  these  facts  it  is  assumed 
that  the  two  kinds  of  chlorophyll  concerned  differ  in  that  one  re- 
mains green  (G)  and  the  other  turns  yellow  (F).  It  is  further 
assumed  that  the  inheritance  of  these  conditions  in  the  cotyledons 
is  through  the  cytoplasm  or  chromatophores  and  not  through 
the  nucleus.  In  the  case  of  color  of  seed-coat  a  Mendelian  factor 
pair  is  involved.  When  H  is  present  it  prevents  the  chlorophyll 
(Y)  in  the  seed-coat  from  changing  to  yellow.  When  this  factor 
is  absent  the  small  letter  h  is  used. 

Table  XXXI  II.  taken  from  Terao  illustrates  four  possible 
combinations.  (G)  and  (Y)  are  transmitted  only  through  the 
cytoplasm  of  the  egg  cell. 

TABLE  XXXIII.  —  INHERITANCE  OF   COTYLEDON  AND  SEED-COAT  COLOR 


Parents  

IN  SOYBEAN  CROSSES 

CROSSING  No.  1 

.  (G)HH9  X  (Y)hhtf 

CROSSING  No.  2 

(G)HH9  X  (Y)HHc? 

Cotyledons  
Seed-coats.  .  .  . 

green              yellow 
green              yellow 

green              yellow 
green              green 

(G)Hh 

(G)HH 

Cotyledons  
Seed-coats.  .  .  . 

green 
green 
(G)HH      (G)Hh      (G)hh 

green 
green 
(G)HH 

Cotyledons  .  .  . 
Seed-coats.  . 

25  per      50  per      25  per 
cent.        cent.        cent. 
green       green       green 
ereen       ereen       ereen 

100  per 
cent, 
green 
ereen 

1  Black  and  brown  pigments  also  appear  in  the  seed-coats  of  certain 
varieties.  These  pigments  are  entirely  independent  of  the  green  and  yellow 
colors  but  they  make  the  green  and  yellow  colors  indistinct. 


148 


BREEDING  CROP  PLANTS 


38ING    No.   3 

CROSSING  No.  4 

?  X  (G}HH<? 

(Y)HH9  X  (G)HH<? 

r             green 

yellow              green 

r             green 

green              green 

(Y)Hk 

(Y)HH 

yellow 

yellow 

green 

green 

(Y)Hh     (Y)hh 

(Y)HH 

50  per      25  per 

100  per 

cent.        cent. 

cent. 

yellow     yellow 

yellow 

green      yellow 

green 

Parents :.  ...       (Y)hh? 

Cotyledons yellow 

Seed-coats yellow 

F, 

Cotyledons 

Seed-coats 

F2 (Y)HH 

25  per 
cent. 

Cotyledons. yellow 

Seed-coats green 


The  inheritance  of  color  of  pubescence  of  soybeans  is  simple. 
The  factor  for  tawny  color  is  allelomorphic  and  dominant  to  the 
factor  for  gray  Segregation  for  color  of  seed  and  color  of 
flower  occurred  in  natural  hybrids  noted  by  Piper  and  Morse 
(1910).  The  number  of  plants  observed  was  not  sufficiently 
large  to  determine  the  factors  involved. 

Breeding. — Pure-line  selections  of  soybeans  have  been  made 
on  the  basis  of  oil  content,  yield  (both  of  seed  and  forage), 
persistence  of  leaves,  and  other  economic  characters.  Varieties 
like  Wisconsin  Black  retain  their  leaves  green  until  practically 
all  the  pods  are  ripe.  Another  character  of  considerable  impor- 
tance in  the  soybean  is  frost  resistance.  It  has  been  found  in 
trials  at  the  Arlington  Experimental  Farm  near  Washington, 
D.C.,  that  varieties  differ  appreciably  in  this  character  in  both 
early  spring  and  late  fall.  Most  of  the  late  varieties  were  killed. 
This  would  indicate  that  the  hereditary  difference  between 
varieties  in  frost  resistance  is  without  doubt  in  part  a  matter  of 
the  degree  of  maturity  which  the  plants  have  reached  at  the 
time  of  frost.  Considerable  artificial  hybridizing  has  been  done 
by  Morse  of  the  United  States  Department  of  Agriculture. 
While  soybeans  have  been  grown  in  the  orient  since  ancient 
times,  their  general  growth  in  the  United  States  and  Europe  is 
comparatively  recent.  As  a  consequence  investigation  with 
this  crop  has  not  proceeded  much  beyond  the  stage  of  variety 
testing  and  strain  isolation.  Then,  too,  there  are  so  many 
varieties  of  different  habits  of  growth,  that  it  has  been  possible 
to  find  a  variety  adapted  to  almost  any  locality.  As  the  real 
value  of  the  soybean  becomes  more  generally  appreciated,  it  will 
undoubtedly  receive  more  attention  from  the  breeding  stand- 
point. 


COWPEAS,  SOYBEANS,  AND  VELVET  BEANS  149 

VELVET  BEAN  (Stizolobium) 

Origin. — Although  little  is  known  of  the  early  history  of  the 
velvet  bean  it  is  thought  that  it  is  a  native  of  India.  The 
Florida  velvet  bean  (Stizolobium  deeringianum)  was  introduced 
into  Florida  previous  to  1875  and  has  never  been  grown  much 
farther  north  because  of  climatic  limitations.  Southern  Georgia, 
Alabama,  Mississippi,  and  Louisiana  mark  the  northern  limits 
of  this  thrifty,  vigorous  growing  legume.  Cultivated  varieties  of 
related  species  of  Stizolobium  have  been  found  in  the  countries 
surrounding  the  Indian  Ocean.  The  most  important  of 
these  is  the  Lyon  bean  (S.  niveum).  Hybridization  between 
this  form  and  the  Florida  velvet  bean  has  produced  many 
different  types,  some  of  which  resemble  other  species  of  Stizolo- 
bium. From  this  fact,  Piper  has  suggested  that  possibly  all 
cultivated  forms  of  Stizolobium  belong  to  a  single  species. 

Important  Characters  and  Inheritance. — The  Florida  velvet 
bean  is  an  annual  of  extremely  vigorous  growth.  Its  branched, 
vine-like  stems  sometimes  reach  a  length  of  from  30  to  50  ft. 
The  leaves  are  large  and  compound,  bearing  ovate  leaflets.  The 
flowers,  which  are  dark  purple  (white  in  some  species),  are  borne 
in  long  racemes.  The  most  important  parts  of  the  plant  from 
a  feeding  standpoint  are  the  pods,  together  with  their  seeds. 
Mature  pods  carry  from  three  to  five  marbled  brown  and  gray 
seeds.  The  pods  are  somewhat  constricted  between  the  seeds 
and  are  covered  with  a  velvety  pubescence.  Another  important 
agronomic  character  is  dehiscence  of  pod.  The  Lyon,  which  has 
pods  nearly  free  from  hair,  scatters  its  seed  when  ripe,  the 
Florida  velvet  bean  does  not.  Pods  of  different  varieties  also 
differ  in  the  degree  of  susceptibility  to  rot  when  in  contact  with 
moist  soil.  The  pods  of  Yokohama  velvet  bean,  from  Japan, 
decay  very  easily. 

Belling,1  of  the  Florida  Agricultural  Experiment  Station, 
has  made  a  study  of  the  inheritance  of  some  of  the  characters  of 
the  velvet  bean.  He  crossed  the  Florida  velvet  bean  extensively 
with  Lyon  bean  and  to  a  lesser  extent  with  Yokohama  and  China 
velvet  beans.  The  Florida  bean  has  a  pubescence  of  whitish 
stiff  hairs  on  its  leaf  buds  and  young  shoots  while  the  ripe  pods 
are  covered  with  brownish  black,  woolly,  flattened  hairs  mixed 
with  a  few  stiff  hairs.  These  hairs  average  1  mm.  in  length. 

1  See  BELLING  (1912a,  1913,  1914a,6,  1915a,6). 


150  BREEDING  CROP  PLANTS 

The  Lyon  bean  has  a  whitish  stiff  pubescence  on  its  young  shoots, 
leaf  and  calyx.  The  hairs  on  the  pods  form  a  fine  down  and 
average  0.5  mm.  in  length.  The  FI  was  covered  with  irritating 
hairs.  The  hairs  on  the  pods  were  about  1.5  mm.  long.  These 
contain  a  gummy  substance  in  the  hollow  points  and  readily 
pierce  the  human  skin,  causing  an  irritation  lasting  several 
minutes.  In  F2  about  nine-sixteenths  of  the  plants  bore  stinging 
pods  (long  stiff  hairs  which  pierce  the  skin).  Some  were  more 
developed  than  in  FI.  Two  factors  are  necessary  for  the  produc- 
tion of  stinging  pods.  One  of  these  factors,  B,  is  contained  by 
the  Lyon  bean  while  C  is  contained  by  the  Velvet  bean.  Color 
of  pubescence  showed  segregation  in  F2,  giving  13  whitish  to 
3  black  pubescent  plants.  The  dehiscence  of  pods  behaved  as 
a  dominant.  Most  of  the  pods  on  the  FI  plants  burst  open  when 
mature.  In  the  F2  generation  segregation  occurred.  Long 
pods  crossed  with  short  pods  gave  approximately  a  3:1  ratio  in 
the  second  generation  although  minor  factors  for  pod  length  were 
discovered.  In  the  inheritance  of  seed  color  it  has  been  suggested 
that  three  factors  are  concerned,  each  of  which  produces  some 
mottling  even  when  heterozygous  and  in  the  absence  of  the  two 
other  factors.  Purple  color  appears  in  the  Florida  velvet  bean 
on  the  under  surface  of  the  first  pair  of  simple  leaves,  on  the 
stems  as  a  mark  on  the  leaf  axil,  on  the  wings  and  standard  and  on 
the  stems  and  petioles  on  the  side  exposed  to  the  sun;  while  the 
Lyon  lacks  the  purple  color.  Purple  color  proved  dominant  in 
FI  and  a  3:1  ratio  was  obtained  in  F2,  only  a  single  factor  being 
involved.  The  characters,  time  of  flowering,  size  of  flower 
clusters,  and  size  of  plant  gave  unmistakable  evidence  of  segre- 
gation in  the  second  generation.  Each  of  the  crosses  Florida 
x  Lyon,  Lyon  x  Florida,  and  Florida  x  Yokohama  produced  about 
50  per  cent,  pollen  sterility  in  the  FI  generation.  Aborted  ovules 
were  found  on  plants  showing  pollen  sterility.  Belling  satis- 
factorily explained  the  results  by  postulating  two  factors,  K  pres- 
ent in  Florida,  and  L  present  in  Lyon  and  Yokohama.  The 
presence  of  either  K  or  L,  but  not  both,  gave  rise  to  normal 
pollen  and  ovules.  Combinations  of  KL  or  kl  in  the  gametes 
resulted  in  pollen  or  ovule  sterility. 

Mutations. — Coe  (1918)  has  attributed  the  origin  of  early 
maturing  velvet  beans  to  mutations.  C.  Chapman  and  R.  W. 
Miller,  both  of  Georgia,  and  H.  L.  Blount  of  Alabama,  separately 
discovered  early  maturing  mutants  growing  in  fields  planted  to 


COWPEAS,  SOYBEANS,  AND  VELVET  BEANS  151 

corn  and  Florida  velevt  beans.  Chapman's  selection  has  been 
increased  and  distributed  under  the  names  ''Georgia"  and  "Hun- 
dred-Day Speckled."  This  variety  requires  120  to  130  days  to 
mature.  The  "Alabama"  variety,  which  matures  in  170  to  180 
days,  or  about  two  months  earlier  than  the  Florida  velvet  bean, 
was  developed  from  an  early  maturing  plant  observed  by  Blount. 

The  discovery  of  these  early  varieties  has  greatly  increased 
the  acreage  of  velvet  beans  by  making  it  possible  to  grow  them 
farther  north.  In  1914  less  than  1,000,000  acres  were  grown, 
whereas,  in  1917,  over  5,000,000  acres  were  given  to  this  crop. 

Another  mutation1  which  is  of  unusual  interest  because  of 
the  long  viny  habit  of  growth  of  the  velvet  bean,  is  the  bush 
form  discovered  recently  in  the  Alabama  variety.  The  appear- 
ance of  the  bush  type  has  been  found  in  other  normally  twining 
beans  such  as  the  common  bean,  the  Lima  bean,  the  hyacinth 
bean,  and  the  soybean.  The  above  mentioned  bush  or  "bunch" 
velvet  bean  was  discovered  by  R.  Beasley  on  his  farm  near  Kite, 
Ga.  He  carefully  saved  the  seed  of  a  single  plant  in  1914  and 
from  the  resultant  crop  grown  in  1915  obtained  about  50  bu. 
The  United  States  Department  of  Agriculture  is  introducing 
this  variety  into  various  localities  of  the  Southern  United  States. 

For  some  purposes  the  bush  variety  possesses  distinct  advan- 
tages. For  instance  when  grown  with  corn  it  has  no  tendency 
to  twine  around  the  corn  stalks  and  pull  them  down.  It  is  also 
better  suited  for  use  as  a  hay  crop.  In  appearance  of  pods  and 
seeds,  ability  of  pods  to  resist  decay  when  on  the  ground,  and 
time  required  to  mature,  the  mutant  is  practically  identical  with 
the  Alabama  variety. 

Breeding. — Some  progress  has  been  made  in  the  improvement 
of  the  Florida  velvet  bean  by  hybridization  and  selection  at  the 
Florida  experiment  station.  A  bean  is  desired  which  will  give 
a  maximum  yield  of  forage  and  seed  of  desirable  quality.  Plants 
with  bristle-like  pubescence  or  small  seeds  with  thick  hulls  are 
undesirable.  Dehiscent  pods  and  also  those  which  decay  readily 
when  lying  on  moist  soil  should  be  avoided.  An  earlier-maturing 
strain  has  been  sought  by  crossing  the  Florida  velvet  bean,  which 
requires  about  200  days  to  mature,  with  Yokohama,  which  re- 
quires about  120  days.  It  is  of  interest  to  point  out  that  from 
one  cross  between  late  varieties  (Florida  x  Lyon)  a  segregate  was 

1  The  following  information  was  furnished  by  the  courtesy  of  DR.  C.  V. 
PIPER. 


152  BREEDING  CROP  PLANTS 

isolated  that  matured  a  month  earlier  than  either  parent. 
This  promising  strain  called  Osceola  has  found  considerable 
favor  in  the  south.  Another  segregate,  a  variety  called  Wakulla, 
obtained  from  the  same  cross,  matures  in  approximately  120  days. 
This  strain  has  an  undesirable  character  in  that  it  shatters  its 
seed  when  ripe.  The  material  available  furnishes  an  opportunity 
to  obtain  further  improvement  by  crossing  and  selections. 


CHAPTER  XII 

FLAX  AND  TOBACCO 

FLAX 

Flax  has  been  reported  to  have  been  grown  by  the  Lake 
Dwellers  of  Switzerland  as  early  as  4,000  to  2,000  years 
B.C.  (Chapter  I).  Although  the  Egyptians  and  Hebrews  used 
flax  to  make  clothing  in  very  ancient  times,  little  is  known  of  the 
origin  of  our  present  cultivated  varieties. 

Species  Crosses. — Tammes  (1911,  1915,  1916)  has  made  some 
interesting  genetic  studies  of  flax  species  crosses.  Reciprocal 
crosses  were  made  between  cultivated  varieties  of  Linum  usitatis- 
simum  and  the  wild  species  L.  perenne,  austriacum,  narbonnenese, 
grandifiorum,  and  angustifolium.  No  seeds  capable  of  germinat- 
ing were  obtained  except  in  the  angustifolium  cross.  This  was 
considered  a  good  cause  for  believing  thatL.  angustifolium  has  the 
best  right  of  any  of  the  wild  species  to  be  considered  the  ancestral 
form  of  cultivated  flax.  This  wild  species  differs  from  the  common 
cultivated  varieties  in  that  the  seeds  and  capsules  are  smaller,  the 
edges  of  the  partition  walls  of  the  capsule  are  hairy,  and  the  capsules 
open  at  maturity.  In  general,  crosses  between  hairy  and  glabrous 
races  showed  dominance  of  the  hairy  condition  in  FI  and  a  segre- 
gation of  3  hairy  to  1  glabrous  in  F2.  The  open  type  of  capsule 
was  imperfectly  dominant  in  F\t  i.e.,  the  capsules  did  not  open 
as  widely  as  in  the  open  parent.  Segregation  occurred  in  F2. 
Parental  types,  i.e.,  homozygous  open  and  homozygous  closed 
lines,  were  produced  in  later  generations.  Three  or  four  factors 
were  necessary  to  explain  results. 

Interrelation  of  Factors  for  Flower  and  Seed-Colors. — Care- 
ful studies  have  been  made  of  the  interrelation  in  inheritance 
of  various  flax  characters  (Tammes  1911,  1914,  1915,  1916). 
The  results  were  carefully  analyzed.  Three  factors  called  A,  B, 
and  C,  were  shown  to  be  necessary  for  the  production  of  dark 
blue  flowers.  B  and  C  together  produce  light  blue  flowers,  and 
A  is  an  intensification  factor  which  in  the  presence  of  B  and  C 
produces  dark  blue  flowers.  When  C  is  homozygous  in  the  pre- 
sence of  B,  the  veins  of  the  petal  are  darker  than  the  rest  of 
the  petal.  The  veins  are  the  same  color  as  the  rest  of  the  petal 
when  C  is  heterozygous  in  the  presence  of  B,  B  and  A  give  the 

163 


154 


BREEDING  CROP  PLANTS 


FIG,  34. — Structure  of  flowers  of  flax. 


FLAX  AND  TOBACCO 


155 


TABLE  XXXIV. — SECOND  AND  THIRD  GENERATION  OF  A  CROSS  BETWEEN 

A  WHITE  FLOWERED  VARIETY  WITH  BLUE  ANTHERS  AND  BROWN  SEED 

(A ABB)  AND  A  CRINKLED  WHITE  VARIETY  WITH  YELLOW 

ANTHERS  AND  YELLOW  SEED  (AACC) 
In  this  table 

d.bl.flr.         =  dark  blue  flower 

l.bl.flr.          =  light  blue  flower 

w.  =  white  flower 

c.w.  =  crinkled  white  flower 

br.s.  =  brown  seed 

y.s.  =  yellow  seed 

bl.st.  =  blue  stamens 

y.st.  =  yellow  stamens 

with  v.         =  with  darker  veins  than  the  remainder  of  the  petal 

without  v.    =  with  veins  of  the  same  color  as  the  body  of  the  petal 


Fz  expected 

obtained 

Fy  expected 

obtained 

1  AABBCC 
d.bl.flr.  with  v., 
bl.st.,  br.s. 
2  AABbCC 
as  above 

3 

213 

Only  d.bl.flr.  with  v.,  bl.st.,  br.s. 

d.bl.flr.  with  v.,  bl.st.,  br.s  3 
c.w.,  y.st.,  y.s  1 

409     plants    in 
several     fami- 
lies. 
60 
21 

2  AABBCc 

d.bl.flr.  without  v., 
bl.st.,  br.s. 

. 

0 

397 

d.bl.flr.  with  v.,  bl.st.,  br.s  1 
d.bl.flr.  without  v.,  bl.st.,  br.s.  .   2 
w.flr.,  bl.st.,  br.s  1 

4 
8 
2 

4  AABbCc 

d.bl.flr.   with  v.,  bl.st.,  br.s   ...   3 

31 

as  above 

d.bl.flr.  without  v.,  bl.st.,  br.s.  .   6 
w.flr.,  bl.st.,  br.s        3 

63 
34 

c.w.flr.,  y.st.,  y.s  3 
w.flr.,  y.st.,  y.s  1 

23 

9 

1  AABBcc 
w.flr.,  bl.st.,  br.s. 

2  AABbcc 
as  above 

3 

203 

to  breed  true. 

w.flr.,  bl.st.,  br.s  3 
w.flr.,  y.st.,  y.s  1 

1,317  plants  in 
several     fami- 
lies. 
1,271 
361 

1  AAbbCC 
c.w.flr.,  y.st.,  y.s. 
2  AAbbCc 

1 

3 

167 

to  breed  true, 
c  w  flr  ,  y  st  ,  y  s                               3 

395 
obtained   such 

c.w.flr.,  y.st.,  y.s. 

w  flr  ,  y  st  ,  v  s                                     1 

segregation. 

1  AAbbcc 
w.flr.,  y.st.,  y.s. 

}\ 

74 

to  breed  true. 

402 

DISCRIPTION  OF  FIG.  34. 

1.  Single  flower — a,  calyx;  6,  corolla. 

2.  Branch  showing — a,  seed;  &,  calyx;  c,  flower  just  after  blooming;  d,  bud. 

3.  Calyx  and  corolla  removed  to  show  sexual  organs  in  position — a,  anther; 
6,  filament;  c,  stigma;  d,  one  of  5  divisions  of  style;  e,  ovary. 

4.  6.  Cross  and  longitudinal  section  of  ovary. 

5.  Ovary,  stigma  and  5-lobed  style. 

7.  Cross  section  of  anther. 

8.  Anther. 

Size:   1,  about  5n;  2,  about  n;  3,  nearly  4n;  4-8,  greatly  enlarged. 


156  BREEDING  CROP  PLANTS 

same  result  when  heterozygous  as  when  homozygous.  C  alone 
or  with  A  gives  wrinkled  petals  and  reduces  the  number  of  seeds 
which  set  per  capsule  and  induces  lower  viability  of  seeds.  B 
prevents  the  above  action  of  C.  B  alone  or  in  the  presence  of 
A  and  C  produces  blue  anthers  and  brown  seeds.  When  B  is 
absent  the  seeds  and  anthers  are  yellow.  Table  XXXIV  gives 
the  result  of  one  of  several  similar  studies.  The  tabular 
presentation  shows  how  carefully  these  studies  were  carried  out. 

Inheritance  of  Size  Characters. — Studies  of  length  of  seed  were 
made  with  crosses  of  the  wild  angustifolium  and  cultivated 
varieties  (Tammes)  as  well  as  with  crosses  between  cultivated 
varieties.  Seeds  were  of  intermediate  size  in  FI  and  segregation 
occurred  in  F2.  The  number  of  individuals  grown  was  not  large 
and  the  parental  forms  were  not  always  again  obtained.  From 
two  to  four  multiple  factors  are  necessary  to  explain  results. 

Length  and  breadth  of  petal  were  also  studied.  Three  forms 
were  used,  a  small-petalled  white-flowered  variety  with  a  petal 
breadth  of  3.3  mm.,  the  common  varieties  with  a  breadth  of 
7  mm.  and  an  Egyptian  cultivated  blue-flowered  variety  with  a 
mean  breadth  of  petal  of  13.4  mm.  In  the  cross  between  Egyp- 
tian blue  and  common  white  the  factors  for  color  of  flower 
and  seed  and  for  size  of  seed  were  apparently  inherited  indepen- 
dently. Breadth  of  petal  ranged  from  one  parent  to  the 
other  in  F%.  Several  factors  for  size  of  flower  were  necessary 
to  explain  results.  The  common  blue  with  a  petal  breadth 
of  7  mm.  was  crossed  with  the  small-petalled  white  with  an  average 
breadth  of  3.3  mm.  In  F2  all  blue-flowered  segregates  agreed  in 
size  with  the  blue  parent  and  all  white-flowered  segregates  had 
small-sized  petals.  The  cross  between  Egyptian  blue  and  the 
small-petalled  white  gave  blue-flowered  races  with  petals  of  inter- 
mediate size  in  FI  and  segregation  for  flower  color  in  F2.  The 
blue-flowered  segregates  gave  a  larger  average  breadth  of  petal 
than  the  white  segregates.  Three  hundred  plants  of  each  color 
were  examined. 

TABLE  XXXV. — CORRELATION  BETWEEN  COLOR  OF  COROLLA  AND  BREADTH 
OF  PETAL  IN  THE  F«  GENERATIONS  OF  FLAX  CROSSES 

RANGE,  MM.     AVERAGE,  MM. 

300  blue-flowered  plants 5.7-16.2  10.8 

300  white-flowered  plants 2. 1-10.4  4.6 

Parent  Egyptian  blue 10.5-16.2  13.4 

Parent  white-flowered . .  2.1-4.2  3.3 


FLAX  AND  TOBACCO 


157 


These  results  were  explained  by  supposing  that  the  small- 
petalled  white  flax  and  the  common  varieties  have  the  same  factors 
for  breadth  of  petal,  C,  one  of  the  color  factors,  when  alone 
or  in  the  presence  of  A  is  an  inhibition  factor  for  flower  size. 
B,  when  present,  prevents  the  action  of  C. 

Wilt  Resistance  in  Flax. — When  flax  is  grown  for  several 
years  on  the  same  soil,  a  heavy  infection  of  Fusarium  lini  often 
results,  and  complete  crop  failure  may  occur.  Bolley,  as  early  as 
1901,  pointed  out  the  true  nature  of  the  disease  and  devised 


FIG.  35. — Selected  and  non-selected  flax  on  wilt  sick  soil.  Right  foreground, 
non-selected  flax  killed  by  wilt;  left  foreground,  selected  flax;  left  background, 
non-selected;  right  background,  selected.  University  Farm,  St.  Paul,  Minn., 
1918.  (After  Stakman,  et  al.,  1919.) 

methods  for  its  control.  Seed  treament  and  crop  rotation  were 
shown  to  be  beneficial  as  aids  in  the  control  of  wilt.  Seed 
selection,  however,  proved  the  most  efficient  control  measure. 
In  general,  Bolley  (1903,  1909)  found  that  two  or  three  years' 
selection  under  disease  conditions  were  necessary  in  order  to 
isolate  a  resistant  variety.  Both  individual  and  mass  selection 
methods  were  used.  Similar  studies  carried  on  at  the  Minnesota 
Station  (Stakman,  et  al,  1919)  have  confirmed  Bolley's  results. 
One  of  the  peculiar  results  of  this  work  is  the  discovery  that 
resistant  varieties  lose  their  resistance  after  they  have  been 


158  BREEDING  CROP  PLANTS 

grown  for  several  years  on  disease-free  soil.  Whether  this 
behavior  is  a  gradual  decrease  in  resistance  of  the  host  which  is 
roughly  proportional  to  the  length  of  time  which  the  resistant 
variety  has  grown  on  wilt-free  soil  or  a  more  or  less  sudden 
change  which  appears  after  two  or  three  years  is  as  yet  unknown. 
The  possibility  that  varietal  and  strain  differences  are  due  to  the 
heterozygous  condition  must  not  be  overlooked. 

As  an  aid  to  seed  selection  in  avoiding  wilt,  early  planting 
is  advocated.  When  planted  early,  a  susceptible  variety  will 
often  partially  escape  the  serious  effects  of  wilt.  Likewise,  a 
resistant  variety  frequently  appears  entirely  wilt  free  when 
planted  early,  while  a  later  planting  may  show  partial  infection. 

Tisdale  (1916,  1917)  has  made  important  contributions  to 
the  nature  and  inheritance  of  wilt  resistance.  A  high  tem- 
perature proved  to  be  an  especially  favorable  agent  in  over- 
coming resistance.  The  fungus  penetrates  the  flax  plant  through 
the  stomata  of  seedlings,  the  root  hairs,  or  the  young  epidermal 
cells.  In  the  resistant  plant,  the  fungus  on  entering  stimulates 
cork  wall  formation  of  cells  adjacent  to  those  attacked,  which 
prevents  further  invasion.  Infection  of  resistant  plants  by 
artificial  inoculation  of  greenhouse  or  field  cultures  of  Fusarium 
lini  did  not  occur  in  43  trials.  Check  infections  of  susceptible 
plants  gave  22  successful  inoculations  out  of  47  trials.  Tube 
cultures  gave  considerable  infection  of  resistant  plants  although 
the  resistance  was  marked  when  these  were  compared  with  tube 
cultures  of  susceptible  strains. 

The  inheritance  of  wilt  resistance  was  studied.  A  great 
difference  in  the  individuality  of  plants  of  the  same  strain  with 
respect  to  resistance  was  shown  by  their  offspring.  Wide  varia- 
tion in  appearance  of  FI  progeny  from  different  crosses  of  sus- 
ceptible and  resistant  plants  of  the  same  strains  was  obtained. 
Segregation  occurred  in  F%.  A  part  of  the  lack  of  uniformity  of 
results  may  be  explained  by  varying  environmental  conditions. 
Tisdale  believes  inheritance  results  can  be  explained  by  multiple 
factors. 

Methods  of  Breeding. — The  flax  plant  is  grown  for  either 
seed  or  fiber.  Varieties  range  in  height  from  approximately 
1J^2  to  more  than  3  ft.  Aside  from  differences  in  inheritance,  the 
thickness  of  planting  strongly  influences  the  habit  of  growth. 
The  fiber  crop  is  largely  produced  in  the  Old  World,  while 
Argentine  and  the  United  States  are  among  the  leaders  in  seed 


FLAX  AND  TOBACCO  159 

production.     Methods  of  breeding  for  seed  or  fiber  flax  are 
essentially  the  same  as  with  the  small  grains. 

TOBACCO 

The  Genus  Nicotiana. — The  tobacco  genus,  Nicotiana,  has 
been  divided  by  earlier  workers  into  four  sections:  Tabacum, 
Rustica,  Petunioides,  and  Polidiclia  (Don,  1838).  More  recently 
the  latter  two  sections  have  been  combined  (East,  1912a  and 
Setchell,  1912).  East's  conclusions  were  reached  by  crossing 
N.  Bigelovii,  of  the  Petunioides  section  with  N.  quadrivalvis, 
which  was  formerly  placed  in  Polidiclia  section.  N.  quadrivalvis 
produces  four-celled  capsules  and  is  a  smaller  plant  than  N. 
Bigelovii.  As  the  FI  hybrid  was  entirely  fertile,  there  seems  no 
good  reason  for  placing  these  forms  in  different  sections.  The 
four-celled  capsule  proved  to  be  a  partially  dominant  character. 

From  the  standpoint  of  the  student  of  plant  genetics  the 
Nicotiana  genus  is  especially  favorable  material.  Some  of  the 
reasons  are: 

1.  Tobacco  may  be  self -fertilized  artificially  with  ease  and  the 
technic  of  crossing  is  very  simple. 

2.  Each  plant  produces  a  large  number  of  seeds  and  the  seed  is 
viable  for  many  years. 

3.  There  are  a  large  number  of  varieties  which  are  entirely 
fertile  inter  se.     These  furnish  especially  favorable  material  for  a 
study  of  quantitative  characters. 

4.  The  different  species  furnish  very  favorable  material  for  a 
study  of  sterility.     Different  crosses  furnish  FI  generations  which 
differ  from  each  other  in  sterility.     The  range  extends  from 
species  crosses  which  give  no  viable  seed  and  from  completely 
sterile  FI  crosses,  to  entirely  fertile  ones. 

The  Tabacum  section  is  represented  by  numerous  varieties 
of  the  species  Nicotiana  tabacum.  These  are  natives  of  the 
New  World.  All  commercial  tobacco  grown  in  the  United 
States  belongs  to  this  species. 

The  Rustica  section  includes  all  the  yellow-flowering  species 
and  varieties.  These  are  of  commercial  importance  in  some 
countries.  In  India  for  example,  they  are  successfully  grown 
commercially  and  for  some  purposes  prove  more  desirable  than 
the  tabacum  varieties  (Howard,  et  al,  1910  b,c).  Among  these 
rustica  forms  are  three  groups;  (a)  one  in  which  the  pistil  is 


160  BREEDING  CROP  PLANTS 

longer  than  the  stamen  and  therefore  which  must  be  artificially 
pollinated  by  hand  or  crossed  by  the  aid  of  insects;  (b)  an  inter- 
mediate type;  and  (c)  forms  in  which  the  stamens  and  pistil 
are  so  arranged  that  self-fertilization  is  the  usual  rule. 

The  Petunioides  section  contains  numerous  varieties  and 
species.  Many  of  these  are  grown  as  ornamental  flowering  types. 

Parthenogenesis. — Parthenogenesis,  meaning  the  production 
of  viable  seed  without  pollination,  was  shown  by  Goodspeed 
(1915)  to  occur  in  N.  tabacum,  variety  Cuba.  Under  normal 
conditions  its  occurrence  is  rare.  Wellington  (1913)  did  not 
find  parthenogensis  in  a  considerable  series  of  experiments 
and  with  numerous  treatments  under  greenhouse  conditions. 
Several  species  as  well  as  several  commercial  varieties  of  N. 
tabacum  were  used  in  this  study.  Howard  (1913)  states  that 
parthenogenesis  in  N.  tabacum  does  not  occur  under  normal 
but  may  occur  under  abnormal  field  conditions,  at  Pusa, 
India. 

Sterility. — Studies  of  crosses  between  N.  tabacum  varieties  and 
N.  sylvestris,  which  belongs  to  the  Petunioides  section,  have  been 
made  by  Goodspeed  and  Clausen  (1917).  The  FI  generation 
proved  to  be  nearly  sterile,  although  a  few  apparently  normal 
pollen  grains  were  produced.  These  could  not  be  caused  to 
germinate  in  their  own  stigmatic  fluid  or  in  other  media.  A  few 
normally  maturing  ovules  capable  of  fertilization  were  produced 
by  theFi  plants.  If  the  plants  were  kept  under  poor  cultural 
conditions  and  the  flowers  pollinated  by  their  respective  parents 
approximately  1  per  cent,  of  the  number  of  seeds  normally  pro- 
duced was  obtained.  If  back-crossed  with  the  sylvestris  parent, 
practically  10  per  cent,  of  the  offspring  of  the  seeds  produced  are. 
pure  sylvestris.  When  crossed  with  tabacum,  part  of  the  plants 
from  the  seeds  produced  seem  to  be  of  normal  tobacco  type  and 
are  fertile;  others  resemble  tabacum  but  are  sterile.  The  FI 
plants  closely  resemble  the  particular  variety  of  N.  tabacum 
which  is  used  as  one  of  the  parents. 

Studies  of  self-sterility  in  tobacco  crosses  have  been  made  by 
East  (1919a,6,c).  East  and  Park  (1917,  1918)  studied  crosses 
between  N.  Forgetiana  and  N.  alata  which  are  self-sterile,  and 
N.  Langsdorffii,  a  self -fertile  species.  Alata  and  Forgetiana 
varieties  sometimes  produce  seed  late  in  the  flowering  season, 
although  during  periods  of  rapid  growth  they  are  entirely  self- 
sterile.  The  few  seeds  obtained  under  reduced  cultural  conditions 


FLAX  AND  TOBACCO  161 

from  selfing  these  self-sterile  species  are  spoken  of  as  cases  of 
pseudo-fertility. 

Results  of  crosses  between  self  sterile  and  self  fertile  varieties 
are  given  in  Table  XXXVI: 

TABLE  XXXVI. — INHERITANCE  OF  STERILITY  IN  CROSSES  BETWEEN  SELF- 
FERTILE  AND  SELF  STERILE  TOBACCO  SPECIES 
PABENTS  Fi  F2 

Forgetiana  X  Langsdorffii Self -fertile     144  self-:  37  self- 
vigorous        fertile  sterile 

Alata  X  Langsdorffii ' Self -fertile     162  self-:  38  self- 
vigorous        fertile  sterile 

The  self -fertile  condition  proved  dominant  in  FI  and  a  ratio  of 
approximately  4  self-fertile  to  1  self-sterile  plant  was  obtained 
in  F2.  The  self -sterile  plants  of  F2  proved  self -sterile  in  later 
generation's.  East  explained  these  results  by  a  dominant  factor, 
F,  for  fertility  and  a  subsidiary  factor,  D,  for  pseudo-fertility 
which  exhibits  itself  only  in  the  presence  of  the  factors  for  sterility, 
ff.  This  pseudo-fertility  factor  produces  some  fertility  under 
certain  conditions,  thus  tending  to  lower  the  number  of  self-sterile 
forms. 

East  has  suggested  that  differences  in  the  rate  of  pollen 
germination  are  largely  responsible  for  the  differences  in  the 
length  of  pollen  tubes  from  compatible  and  incompatible  pollina- 
tions. When  self-sterile  plants  are  self-pollinated  the  pollen 
grains  germinate  but  the  pollen  tubes  grow  so  slowly  that  abscis- 
sion of  the  flower  occurs  before  the  pollen  tube  reaches  the  ovary. 

Color  Characters.— Color  of  corolla  has  been  studied  for  Nico- 
tiana  crosses.  East  (19166)  found  that  in  crosses  between  N. 
Langsdorffii  and  N.  alata  there  was  a  dominance  in  F\  of  yellow 
over  white  in  the  color  of  the  corolla.  The  Langsdorffii  parent 
produces  blue  pollen  and  the  alata  yellow.  Reciprocal  crosses 
gave  blue  pollen  in  FI,  although  the  color  was  somewhat  lighter 
than  in  the  blue-pollen  parent.  Results  in  F2  showed  342 
plants  with  blue  pollen  and  100  plants  with  yellow  pollen.  Yel- 
low-pollen plants  bred  true  in  F3.  Here  we  have  a  cross  between 
species  which  exhibits  a  monohybrid  ratio. 

According  to  Allard  (1919a)  N.  tabacum  exhibits  three  distinct 
flower  colors — carmine,  pink,  and  white.     In  crosses  between  car- 
mine and  pink  theFi  was  carmine.     The  FI  pollinated  with  the  car- 
mine parent  gave  all  carmine  colored  progeny  while  the  FI  crossed 
11 


162 


BREEDING  CROP  PLANTS 


with  the  pink  gave  carmine  and  pink  in  a  ratio  of  1:1.  This 
indicates  that  carmine  and  pink  differ  in  one  genetic  factor. 
In  a  cross  of  carmine  and  white  the  FI  was  all  light  carmine.  In 
F2  there  were  54  carmine,  95  light  carmine,  26  dark  pink,  38  light 
pink,  and  65  white.  Some  of  the  extracted  whites  revealed  a 
tinge  of  color.  Crosses  of  extracted  whites  with  pink  gave  32 
carmine  and  62  pink,  showing  that  extracted  whites  sometimes 
carried  a  carmine  factor.  The  factor  relations  are  not  entirely 
clear. 

Quantitative  Characters. — Many  of  the  so-called  size  charac- 
ters of  tobacco  are  of  great  commercial  importance.  For  this  rea- 
son their  mode  of  inheritance  is  of  much  interest  to  the  breeder. 
Extensive  studies  of  inheritance  of  these  size  characters  have 
been  made.  Inheritance  of  leaf  number  will  be  given  as  an 
example  of  a  common  type  of  inheritance  of  size  characters  in 
this  group.  Sumatra,  which  averages  27  leaves,  was  crossed  with 
Broadleaf,  which  gives  an  average  of  19.4  leaves.  The  results 
for  the  parents  and  FI  to  F3  generations  as  obtained  at  the  Con- 
necticut Station  are  given  in  Table  XXXVII  (Hayes,  East,  and 
Beinhart,  1913). 


TABLE  XXXVII. — INHERITANCE  OF  LEAF  NUMBER  IN  CROSS  (403  X  401) 
SUMATRA  X  BROADLEAF 


Number 

Year 
grown 

Gener- 
ation 

Leaves 
of 
parent 

Range  of 
variation 

Total 

Mean 

C.  V. 

403  Sumatra  
403-1  
403-1-2  

1910 
1911 
1912 

Pi 
Pt 
P3 

29 
29 

24-31 
23-31 
21-32 

150 
125 
151 

28.2±0.08 
26.5±0.11 
26.2±0.12 

5.27  +  0.21 
6.  64  ±0.28 
8.28  +  0.32 

401  Broadleaf... 
401-1  
401-1-1  

1910 
1911 
1912 

Pi 

P-L 

Ps 

20 
22 

17-22 
16-22 
17-23 

150 
108 
145 

19.  2  ±0.05 
19  .  1  ±  0  .  08 
19  9+0  07 

5.00  +  0.19 
6.  54  ±0.30 
6  03+0  24 

403  X  401  =  B.. 
B-l  

1910 
1911 

Fi 

Fz 

25 

19-26 
17-32 

150 

2,402 

23.6±0.07 
22  7  +  0  03 

5.51±0.21 
8  99  +  0   11 

£-3  

1911 

F2 

24 

17-35 

1,632 

22  .  5  ±  0  .  03 

9.5]  ±0.10 

#-1-4  

1912 

F3 

25 

16-29 

179 

22.5±0.12 

10  84+0  39 

£-1-7 

1912 

F3 

22 

17-28 

207 

21   5  +  0   10 

10   14  +  0  34 

£-1-8 

1912 

F3 

28 

19-33 

82 

26  3  +  0  20 

10  38  +  0  55 

£-1-10  
B-l-12  
£-1-14  
£-3-5 

1912 
1912 
1912 
1912 

F3 
F3 
F3 
F3 

26 
25 
25 
27 

19-27 
18-30 
19-29 
17-28 

151 
209 
56 
159 

23.1+0.10 
23.7±0.14 
21.8  +  0.14 
21  7  +  0  11 

7.75±0.30 
10.  51  ±0.41 
7.18  +  0.46 
9  45  +  0  36 

£-3-6  
£-3-8  

1912 
1912 

F3 
F3 

28 
25 

16-27 
17-23 

229 

85 

22.5±0.00 
20.6  +  0.12 

8.71+0.27 
8.25  +  0  43 

FLAX  AND  TOBACCO  163 

The  Broadleaf  variety  is  commonly  grown  in  one  section  of  the 
Connecticut  Valley  and  is  especially  valuable  for  cigar  wrappers. 
Sumatra  which  is  an  imported  variety  produces  many  leaves 
per  plant  but  they  are  small.  As  may  be  seen  from  an  examina- 
tion of  the  table,  the  FI  had  an  intermediate  number  of  leaves. 
Segregation  occurred  in  F2  and  selected  F2  plants  gave  F3  families 
which  differed  in  the  average  number  of  leaves.  £-1-14,  showed 
the  lowest  coefficient  of  variability  of  any  F3  family.  Progeny 
of  this  same  F2  plant  were  also  grown  at  another  locality  and 
they  proved  uniform  in  number  of  leaves,  the  calculated  coeffi- 
cient of  variability  being  6.44  ±  0.27.  5-1-10  gave  a  low  coeffi- 
cient of  variability  and  a  mean  leaf  number  which  was  about  the 
same  as  in  the  FI  generation,  i.e.,  intermediate  between  the 
parents. 

A  cross  was  studied  between  Connecticut  Havana,  which  is 
grown  as  a  wrapper  and  binder  tobacco  both  in  the  Connecticut 
Valley  and  in  Wisconsin,  and  Cuban,  a  variety  commonly  grown 
under  shade.  The  parents  and  FI  gave  about  the  same  number 
of  leaves  but  in  F2  there  was  a  great  increase  of  variability,  forms 
being  obtained  with  a  higher  and  lower  leaf  number  than  in  either 
parent.  The  inheritance  of  size  and  shape  of  leaf  was  likewise 
investigated.  The  Cuban  variety  gives  a  short  broad  leaf  and 
the  Havana  a  longer  leaf  which  is  proportionally  narrower  than 
the  Cuban.  Lines  were  obtained  in  F3  which  bred  true,  respec- 
tively, to  the  parental  leaf  shapes. 

East  (1916a)  has  listed  eight  requirements,  most  of  them  inde- 
pendent mathematically,  which  should  be  met  if  size  inheritance 
is  typically  Mendelian,  when  all  populations  succeeding  the 
original  cross  are  obtained  by  growing  progeny  of  single  self- 
fertilized  plants.  These  are: 

"1.  Crosses  between  individuals  belonging  to  races  which  from  long 
continued  self-fertilization  or  other  close  inbreeding  approach  a  homozy- 
gous  condition,  should  give  FI  populations  comparable  to  the  parental 
races  in  uniformity. 

"2.  In  all  cases  where  the  parental  individuals  may  reasonably  be 
presumed  to  approach  complete  homozygosity,  F2  frequency  distribu- 
tions arising  from  extreme  variants  of  the  FI  population  should  be  prac- 
tically identical,  since  in  this  case  all  Ft  variation  should  be  due  to 
external  conditions. 

"3.  The  variability  of  the  F2  population  from  such  crosses  should  be 
much  greater  than  that  of  the  FI  population. 


164  BREEDING  CROP  PLANTS 

"4.  When  a  sufficient  number  of  F2  individuals  are  available,  the 
grandpa  rental  types  should  be  recovered. 

"5.  In  certain  cases  individuals  should  be  produced  in  F%  that  show  a 
more  extreme  deviation  than  is  found  in  the  frequency  distribution  of 
either  grandparent. 

"6.  Individuals  from  various  points  on  the  frequency  curve  of  an  Fz 
population  should  give  Fz  populations  differing  markedly  in  their  modes 
and  means. 


FIG.  36. — A,  N.  data  (jrandiflora;  B,  Fi  of  N.  langsdorfii  X  N.  alata  grandi- 
f.ora;  C,  N.  langsdorfii  (1911);  D  and  E,  extremes  of  the  F2  generation  (1912)  X 
H.  (After  East.) 

"7.  Individuals  either  from  the  same  or  from  different  points  on  the 
frequency  curve  of  an  F%  population  should  give  Fz  populations  of  diverse 
variabilities  extending  from  that  of  the  original  parents  to  that  of  the 
Ft  generation. 

"8.  In  generations  succeeding  the  F*,  the  variability  of  any  family 
may  be  less  but  never  greater  than  the  variability  of  the  population 
from  which  it  came." 

All  of  the  above  eight  conditions  have  been  obtained  in  experi- 
ments and  no  fact  directly  opposed  to  them  has  been  discovered. 

The  quantitative  characters  in  tobacco  which  have  been 
studied  are,  therefore,  typically  Mendelian  in  their  inheritance. 


FLAX  AND  TOBACCO 


165 


A  list  of  these  characters  and  of  the  authority  for  the  inheritance 
is  here  given.  Not  all  papers  on  this  subject  are  included.  Those 
given  show  the  general  behavior  of  many  of  the  characters  in 
inheritance. 

TABLE  XXXVIII. — INHERITANCE  OF  TOBACCO  CHARACTERS  AS  SHOWN  BY 
RESULTS  01  CROSSES 


Character 


Grown  in 


Authority 


Height  of  plant 

Fi 

Jensen,  1907. 

Height  of  plant  
Height  of  plant 

Ft,  ft 

Fi  to  Ft 

Hayes,  1912. 
Howard,  1913. 

Number  of  leaves  
Leaf  size     

Ft  to  F, 
Fi  to  F3 

Hayes,  East,  and  Beinhart,  1913. 
Howard,  1913. 
Hayes,  East,  and  Beinhart,  1913. 

Leaf  shape 

Fl    tO   F-l 

Howard,  1913. 
Howard   1913.     Hayes,  East,  and 

Beinhart,  1913. 

Spread,  length  and  diameter 
of  corolla. 

Sucker  inheritance .  . 
Base  of  leaf. . 


F,  to  F3     Goodspeed,     1912,     1913.     East, 

19166. 
i  to  F3    \  Johnson,  1919. 
to  F3    :  Howard,  1913 


The  results  obtained  in  these  studies  of  tobacco  show  that 
segregation  occurs  in  Fz  for  size  characters  and  that  forms 
similar  to  the  parents  as  well  as  new  sorts  may  be  obtained  in 
later  generations. 

Environment  as  a  Factor  in  Tobacco  Breeding. — It  is  a  matter 
of  common  knowledge  that  environmental  conditions  widely 
modify  the  expression  of  characters.  This  is  particularly  notice- 
able in  tobacco,  where  quality,  size,  and  shape  of  leaf  are  of  such 
marked  importance.  A  belief  has  been  frequently  expressed 
that  environment  causes  a  breaking  of  type.  The  following 
quotation  from  Sham  el  (1910)  emphasized  this  view  for  tobacco: 

"The  writer  believes  that  the  two  efficient  means  of  inducing  varia- 
bility as  a  source  of  new  types  are  change  of  environment  and  crossing. 
So  far  as  the  writer  is  concerned,  the  change  of  environment — usually 
the  growing  of  southern  grown  seed  in  the  north — is  the  most  effective 
means  of  inducing  variability." 

Statements  of  this  nature  have  been  used  as  evidence  that 
environment  modifies  the  characters  of  a  pure  line  by  inducing 


166  BREEDING  CROP  PLANTS 

variability.  A  careful  survey  of  experimental  studies  does  not 
support  this  contention.  The  development  of  the  shade- 
grown  tobacco  industry  in  the  Connecticut  Valley  is  of  interest 
in  this  discussion.  This  shade  method  first  originated  in  Florida 
in  1896  and  was  tried  experimentally  in  Connecticut  through 
cooperation  of  the  Connecticut  Experiment  Station  and  officials 
of  the  Bureau  of  Soils.  In  1900  one  third  of  an  acre  was  grown 
and  the  crop  sold  at  an  average  price  of  72  cents  per  pound. 
A  considerable  acreage  was  grown  in  1901  and  the  crop  sold 
at  public  auction  at  a  much  higher  price  per  pound.  Indis- 
criminate introduction  of  unselected  seed  from  Florida  was 
practiced  and  in  1902  over  700  acres  were  grown  under  shade  in 
Connecticut.  The  result  was  a  disastrous  failure,  owing  to  a 
lack  of  knowledge  of  methods  of  handling  and  to  the  use  of  un- 
selected seed.  By  further  study  of  handling  and  through  careful 
selection  in  which  artificially  self -pollinated  seed  was  saved, 
the  industry  was  placed  on  a  firm  foundation.  This  latter  work 
was  carried  on  by  the  Bureau  of  Plant  Industry  (Stewart,  1908). 
A  knowledge  of  Cuban  methods  shows  that  imported  Cuban  seed 
is  a  mixture  of  many  types.  Some  experiments  have  shown  that 
the  breaking  up  alluded  to  is  an  expression  of  the  different 
hereditary  qualities  of  the  parental  seed  plants.  In  1912  Hassel- 
bring  grew  a  number  of  pure  lines  of  tobacco  in  Michigan  which 
he  had  formerly  grown  in  Cuba.  No  evidence  of  breaking  up  of 
type  was  observed  and  whatever  changes  occurred  in  a  pure  line, 
owing  to  the  new  conditions,  were  uniformly  exhibited  in  all 
plants  of  the  pure  line.  Similar  conclusions  were  reached  from 
the  immediate  introduction  of  individual  seed  capsules  of  different 
tobacco  plants  from  Cuba  and  their  subsequent  growth  under 
shade  in  Connecticut  (Hayes,  1914).  Careful  studies  at  Pusa, 
India,  convinced  the  Howards  (1910a)  that  new  conditions 
did  not  cause  a  breaking  up  of  type.  They  ascribed  the  apparent 
variability  of  new  introductions  to  cross-fertilization,  which 
was  shown  to  occur  frequently  in  tobacco. 

Although  there  have  been  some  differences  of  opinion  as  to 
the  cause  of  variability  of  new  introductions,  there  is  uniformity 
of  belief  regarding  the  methods  of  obtaining  purity  of  type. 
Artificial  self-pollination  gives  uniformity,  and  continued 
self-fertilization  produces  no  harmful  effects.  This  method  was 
strongly  recommended  by  officials  of  the  United  States  Depart- 
ment of  Agriculture  (Shamel  and  Cobey ,  1907)  and  by  the  different 


FLAX  AND  TOBACCO  167 

state  experiment  stations.  The  Howards,  in  India,  likewise 
urged  the  use  of  self -fertilized  seed.  Garner  (1912)  states  that 
several  types  have  been  inbred  by  growing  the  seed  under  bag 
from  six  to  eight  years  without  any  observable  change  in  vigor 
or  habits  of  growth.  These  facts,  together  with  the  studies 
of  inheritance  of  quantitative  characters,  show  that  the  pure- 
line  theory  and  Mendel's  law  furnish  a  reliable  guide  to  tobacco 
breeding  operations. 

As  quality  of  cured  leaf  is  of  such  great  importance  in  tobacco, 
it  is  necessary  that  the  breeder  have  a  thorough  knowledge  of  the 
sort  of  leaf  desired.  Practical  breeding  operations  must  then  be 
carried  on  under  the  soil  and  climatic  conditions  in  which  the  crop 
is  to  be  grown.  An  added  complication  is  the  necessity  of  basing 
the  final  judgment  of  a  particular  selection  upon  the  comparative 
value  of  the  cured  leaf  after  fermentation.  The  difficulties  of 
comparing  numerous  strains,  while  not  insurmountable,  are 
naturally  much  greater  than  for  an  equal  number  of  small  grain 
selections. 

Mutations  in  Tobacco. — The  sudden  appearance  of  giant 
plants  with  abnormally  high  leaf  number  has  been  recorded  in 
the  Sumatra,  Maryland,  Cuban,  and  Connecticut  Havana  varie- 
ties of  N.  tabacum  (Allard,  1916).  These  new  forms  under  field 
conditions  have  a  much  longer  period  of  vegetative  vigor  than 
the  normal  varieties.  Consequently  blossoming  does  not  take 
place  under  ordinary  field  conditions.  Otherwise  the  general 
habit  of  each  of  these  new  types  is  not  very  different  from  the 
normal  variety  from  which  it  was  obtained. 

Two  of  these  new  varieties  of  giant  habit  are  of  some  commer- 
cial importance.  A  short  account  of  their  first  recorded  appear- 
ance together  with  their  cultivation  as  commercial  varieties  will 
be  given.  Giant  plants  were  noted  in  1912  in  the  Cuban  variety 
which  is  grown  under  shade  in  the  Connecticut  Valley.  (Hayes 
and  Beinhart,  1914).  The  history  of  the  normal  Cuban  variety 
from  which  the  giant  type  was  obtained  is  of  interest  (Hayes, 
1915).  Seed  of  the  normal  variety  was  saved  under  bag,  which 
insures  self-fertilization,  from  1904  to  1909  inclusive.  In  1910 
and  1911  seed  was  saved  in  bulk  from  plants  which  were  grown 
under  the  cheese-cloth  cover  used  in  producing  shade-grown 
tobacco,  but  individual  plants  were  not  bagged.  During  the 
period  from  1904  to  1910  no  abnormal  types  were  observed. 
Studies  of  leaf  inheritance  in  the  Cuban  variety  were  made  from 


168  BREEDING  CROP  PLANTS 

1910  to  1914  inclusive.  An  average  of  150  plants  was  carefully 
examined  yearly  and  no  aberrant  types  were  observed. 

In  1912  about  100  acres  were  grown  by  the  Windsor  Tobacco 
Growers'  Corporation  from  seed  saved  in  1911,  and  late  in  the 
season  three  plants  were  discovered  which  had  produced  a  high 
leaf  number  and  showed  no  signs  of  blossoming.  One  of  these 
plants  when  taken  to  the  Connecticut  Experiment  Station  green- 
house produced  72  leaves  and  blossomed  about  January  first. 
Considerable  seed  was  saved  from  this  plant  and  one-third  acre 
of  the  new  type  was  grown  in  1913.  The  plants  were  of  uniform 
appearance.  They  differed  from  the  normal  Cuban  in  having 
leaves  of  a  somewhat  lighter  green,  in  having  but  few  basal 
suckers,  and  in  a  long  continued  period  of  growth;  whereas 
the  normal  Cuban  variety  bears  a  terminal  inflorescence 
after  producing  from  14  to  25  leaves  on  the  main  stem. 
From  25  to  30  acres  have  been  grown  yearly  by  the  same  tobacco 
company.  The  quality  and  yield  of  this  giant  variety  which  has 
been  named  Stewart  Cuban,  have  been  quite  satisfactory.  One  of 
the  great  difficulties  of  growing  these  giant  forms  is  the  extra 
trouble  of  obtaining  seed.  This  difficulty  has  been  overcome  in 
part  by  studies  which  show  that  a  reduction  of  length  of  day  leads 
to  the  production  of  blossoms.  These  studies  will  be  briefly  de- 
scribed after  giving  a  short  history  of  the  Maryland  Mammoth 
type. 

The  Maryland  Narrowleaf  Mammoth  type  first  appeared  in 
1907  in  the  second  generation  of  a  cross  between  two  common 
varieties  of  Maryland  tobacco  (Garner,  1912).  One  hundred 
and  fifty-seven  plants  of  this  new  form  were  grown  in  1908  and 
all  plants  were  of  mammoth  habit.  This  new  variety  has  been 
grown  commercially  since  that  time  and  retains  its  characteristics 
of  high  leaf  number  and  non-blooming  habit  under  normal  field 
conditions.1  Accurate  information  regarding  the  acreage  of 
Mammoth  tobacco  in  southern  Maryland  is  not  available  but 
some  hundreds  of  acres  were  grown  in  1920.  The  chief  limiting 
factor  in  the  acreage  is  the  quantity  of  seed  available.  As  Mary- 
land tobacco  is  harvested  by  cutting  and  spearing  the  stalk, 
there  is  little  additional  cost  in  harvesting  the  giant  type.  The 
Mammoth  variety  will  yield  2,000  Ib.  or  more  per  acre  and  the 

1  Information  kindly  furnished  by  DR.  W.  W.  GARNER,  Physiologist, 
in  Charge  of  Tobacco  and  Plant  Nutrition  Investigations,  B.  P.  I.,  United 
States  Department  of  Agriculture. 


FLAX  AND  TOBACCO  169 

quality  of  cured  leaf  is  superior  to  the  ordinary  varieties.  Com- 
parative yields  show  that  the  Mammoth  variety  yields  20  to 
25  per  cent,  more  than  other  varieties  when  grown  on  produc- 
tive soil.  As  the  Mammoth  variety  has  shorter  internodes  than 
ordinary  varieties  the  leaves  shade  one  another.  This  prevents 
coarse  texture  and  dark  colors  even  on  highly  productive  soil. 
The  ordinary  varieties,  when  grown  on  rich  soils,  yield  dark- 
colored  and  coarse-textured  leaves.  The  value  per  acre  of  the 
Mammoth  tobacco  is  30  to  40  per  cent  higher  than  ordinary 
varieties  (see  Fig.  37). 

Garner  and  Allard  (1920)  have  studied  the  effect  of  relative 
length  of  day  on  growth  and  development  of  plants,  particu- 
larly with  respect  to  sexual  reproduction.  By  placing  a  venti- 
lated, dark  chamber  in  the  field  the  relative  number  of  hours 
of  exposure  to  sunlight  was  controlled  as  desired.  They  found 
that: 

"  Normally  the  plant  can  attain  the  flowering  and  fruiting  stages 
only  when  the  length  of  day  falls  within  certain  limits,  and,  conse- 
quently, these  stages  of  development  ordinarily  are  reached  only  during 
certain  seasons  of  the  year.  In  this  particular,  some  species  and  vari- 
eties respond  to  relatively  long  days,  while  others  respond  to  short  days, 
and  still  others  are  capable  of  responding  to  all  lengths  of  the  day  which 
prevail  in  the  latitude  of  Washington  where  the  tests  were  made." 

In  the  absence  of  a  favorable  length  of  day  for  bringing  into 
expression  reproductive  processes  in  certain  species,  vegetative 
development  may  continue  and  thus  lead  to  the  production  of 
such  varieties  as  Stewart  Cuban  and  Maryland  Mammoth  which 
under  ordinary  conditions  never  reach  the  flowering  stage. 

"Thus,  certain  varieties  or  species  may  act  as  early  or  late  maturing, 
depending  simply  on  the  length  of  day  to  which  they  happen  to  be 
exposed." 

The  Stewart  Cuban  and  Maryland  Mammoth  varieties  of 
tobacco,  as  well  as  several  other  species  were  used  in  a  deter- 
mination of  the  effect  of  reduced  length  of  day  in  forcing  flower- 
ing. In  discussing  the  effects  of  controlling  light  as  a  means  of 
forcing  flowering  in  Maryland  Mammoth,  Garner1  says; 

"Under  a  given  length  of  day  favorable  to  flowering,  this  type  can  be 
1  From  a  letter  written  September  14,  1920. 


170 


BREEDING  CROP  PLANTS 


FLAX  AND  TOBACCO  171 

made  to  produce  any  quantity  of  seed  ranging  from  a  single  pod  up  to 
a  large  inflorescence  by  appropriate  regulation  of  the  quantity  of  soil 
in  which  the  plant  grows." 

Plants  grown  in  12-quart  buckets  produced  large  amounts 
of  seed  when  the  length  of  day  was  shortened  by  placing  the 
plants  in  the  dark  chamber  for  a  part  of  the  normal  day.  A 
control  series  left  out  of  doors  during  the  experiment  began  to 
show  flower  heads  about  the  middle  of  August  (see  Fig.  38). 
Plants  exposed  to  seven  hours  of  light  daily  produced  large  quanti- 


FIG.  38. — Control  series  of  Maryland  Mammoth  tobacco  in  twelve-quart 
buckets  left  out  of  doors  during  the  experiment.  Flower  buds  just  beginning 
to  show  when  photographed,  August  19,  1919.  (Courtesy  of  Garner.} 

ties  of  seed  while  those  exposed  to  twelve  hours  of  light  daily 
grew  larger  but  were  later  in  blossoming  (see  Fig.  39). 

In  southern  Florida  during  the  ordinary  winter  months,  the 
Maryland  Mammoth  behaves  as  ordinary  tobacco,  showing  no 
evidence  of  its  tall  late  habit.  Thus  quantities  of  seed  could 
easily  be  produced  under  these  conditions. 

Allard  (1919)  crossed  normal  varieties  with  the  Mammoth 
type.  The  F\  averaged  somewhat  higher  in  leaf  number  than  the 


172 


BREEDING  CROP  PLANTS 


normal  varieties  but  invariably  blossomed  under  field  conditions 
in  practically  the  same  period  as  ordinary  varieties  of  N.  tabacum. 


FIG.  39. — Front  row  in  twelve-quart  buckets  exposed  to  light  from  9  a.m. 
to  4  p.m.  or  7  hours  daily.  Rear  row  in  twelve-quart  buckets  exposed  to  light 
from  6  a.m.  to  6  p.m.  or  12  hours  daily.  Note  that  latter  are  larger  plants  but 
flowered  considerably  later  than  the  former.  (Courtesy  of  Garner.) 

A  total  of  1820  F%  plants  was  grown  and  439  were  of  the  giant 
habit. 


CHAPTER  XIII 
COTTON  AND  SORGHUM1 

Little  is  definitely  known  of  the  antiquity  and  origin  of  cotton. 
Evidence  has  been  obtained  which  indicates  that  it  was  culti- 
vated in  India  in  1,500  B.C.  and  in  Egypt  1,300  years  later. 
Species  of  cotton  are  indigenous  both  to  tropical  America  and  to 
India.  Because  of  the  extent  of  natural  crossing  (5-13  per  cent.) 
(Balls,  1912)  the  difficulties  of  studying  inheritance  and  of  carry- 
ing on  practical  breeding  operations  are  very  great.  It  seems 
reasonable,  however,  to  consider  this  crop  in  the  self-fertilized 
group,  as  the  extent  of  crossing  leads  to  the  belief  that  continued 
self-fertilization  will  not  give  harmful  results.  It  also  seems 
reasonable  to  conclude  that  deterioration  in  a  selected  variety  is 
largely  the  result  of  natural  crossing.  Methods  of  pedigreed  seed 
production  should,  therefore,  be  developed  to  their  highest 
possible  efficiency. 

Classification  and  Inheritance. — Gossypium  contains  several 
species.  The  two  species  of  cotton  grown  commercially  in  the 
United  States,  upland  (G.  hirsutum)  and  sea-island  (G.  barbadense) 
cross  readily  with  each  other.  The  varieties  cultivated  in  Egypt 
also  belong  to  G.  barbadense,  but  in  India  the  forms  derived  from 
G.  herbaceum  are  chiefly  grown.  Webber  (1905)  was  unable  to 
cross  Aiden  cotton  which  he  classified  as  G.  herbaceum,  with  either 
sea-island  or  upland  varieties.  The  commercial  value  of  cotton 
and  the  separation  into  the  above  species  groups  is  largely  deter- 
mined by  three  characteristics  of  the  fiber:  namely,  length, 
tensile  strength,  and  fineness.  Other  morphological  characters 
have  been  used  in  classification.  These  are  presence  or  absence 
of  fuzz  on  the  seed,  color  of  fiber  and  flower,  form  of  boll  and 
general  habit  of  growth.  The  wide  range  of  environmental  or 
place  effect  exhibited  by  the  cotton  plant  generally,  as  well  as 
heterozygosis  due  to  natural  crossing,  has  made  clear-cut  classi- 
fication difficult.  G.  hirsutum  is  a  vigorous  annual  plant  with  a 

1  While  sorghum  is  botanically  one  of  the  grasses,  yet  from  the  stand- 
point of  the  breeder  it  is  better  treated  with  the  self-fertilized  group  of 
crop  plants. 

173 


174  BREEDING  CROP  PLANTS 

branching  upright  stem  and  a  tap  root  with  numerous  lateral 
branches.     The  leaves  are  alternate,  3  to  6  in.  long,  slightly  less 


FIG.  40. — Upper  left,  flower  of  Upland  cotton,  from  below,  with  bracts 
removed  showing  the  arrangement  of  calyx  lobes,  petals  and  nectaries;  at 
right,  petals;  lower  left  flower  of  cotton  with  one  bract  removed  showing  spirally 
arranged  stamens  and  stigma.  (After  Cook.) 

in  breadth,  the  lower  ones  being  heart  shaped,  the  upper  more 
or  less  three-  or  five-lobed.  The  flowers  are  large  and  showy. 
The  fruit  develops  into  a  pointed  egg-shaped  body  about  the 


COTTON  AND  SORGHUM  175 

size  of  a  small  hen's  egg  and  is  closely  filled  with  seeds.  It  is 
composed  of  from  three  to  five  cells.  When  ripe  the  boll  turns 
brown  and  splits  open  and  the  lint  and  seed  are  exposed. 
The  seeds,  each  about  %  in.  long  and  half  as  wide,  are  covered 
with  lint  and  fine  fuzz.  This  lint,  the  cotton  of  commerce,  is 
from  %  to  IK  m-  long  in  the  ordinary  varieties.  (Wilson  and 
Warburton,  1919).  G.  barbadense  is  distinguished  in  part  from 
G.  hirsutum  by  its  greater  height,  longer  branches,  longer  and 
finer  fiber,  and  seeds  free  from  fuzz.  Egyptian  is  generally 
considered  a  variety  of  (7.  barbadense.  It  is  the  variety  grown 
largely  in  Egypt,  also  under  irrigation  in  Arizona  and  southern 


FIG.  41. — View  of  flower  of  cotton,   from  above,  showing  position  of  petals, 
stigmas  arid  stamens  (natural  size).     (After  Cook.) 

California.  India  cotton  (G.  herbaceum)  has  stems  more  slender 
than  upland,  and  leaves  with  rounded  lobes  and  smaller,  less 
pointed  bolls.  The  lint  is  white,  yellow,  or  brown.  Its  cultivation 
is  confined  to  southern  Asia  (Wilson  and  Warburton,  19 19) .  Balls 
(1908,  1911,  1912)  and  Leake  (1911)  have  investigated  color 
inheritance  of  seed  fuzz,  lint,  anthers,  flowers,  and  sap.  In 
most  cases  the  second  generation  gave  a  mono-  or  di-hybrid  ratio. 
On  the  other  hand,  ratios  were  also  obtained  which  were  not  easily 
explained  on  a  simple  factor  basis.  The  inheritance  of  the  red 
spot  on  the  petals  of  some  varieties  involves  two  factors  with  a 
3:1:1:3  coupling.  Red  spot  on  the  leaf  showed  simple  mono- 


176 


BREEDING  CROP  PLANTS 


hybrid  segregation.  In  some  crosses,  studied  independently  by 
McLendon  (1912)  and  Balls,  between  the  dominant  fuzzy-seeded 
and  the  recessive  smooth-seeded  forms,  evidence  was  found  of  a 
single-factor  difference  as  well  as  a  two-factor  difference.  Balls 
also  discovered  that  long  fiber  was  dominant  to  short  and  that 
but  one  main  factor  difference  existed.  Likewise  a  dominant 
long  petal  crossed  with  a  recessive  short  petal  gave  in  the  F2 
generation  a  3:1  segregation.  In  a  cross  between  Egyptian 
(Abassi)  and  Texas  Upland,  Balls  obtained  transgressive  segre- 
gation in  height  of  plants.  Similar  results  were  secured  in 
studying  the  inheritance  of  date  of  flowering  and  weight  of  seed. 
The  results  of  the  study  of  weight  of  seed  are  of  general  interest. 


0.050  0.100  0.150  Gram 

FIG.  42. — Seed  weights  of  parent  varieties,  King  and  Charara,  and  Fi  and  Ft 
generation  crosses.     (After  Balls.) 

In  a  cross  of  Afifi  and  Truitt,  where  the  mean  seed  weights  of  the 
parents  were  0.105  g.  and  0.135  g.  respectively,  the  weight  of  the 
Fi  was  0.165  g.  Weights  in /^varied  from  0.08  g.  to  0.175  g.  The 
light-seeded  forms  bred  comparatively  true  in  Fz  although  differ- 
ing somewhat  in  means.  The  larger-seeded  types  bred  true  in 
F3  or  segregated,  giving  both  large-and  small-seeded  forms.  An 
illustration  of  this  sort  of  behavior  for  the  parents,  FI  and  F2 
generations,  is  given  in  figure  42  diagrammatically.  As  has  been 
pointed  out,  length  of  lint  is  also  inherited  and  in  some  cases 
segregation  approaches  a  simple  3:1  ratio  with  long  lint  as  the 
dominant  character.  Later  generations  'in  some  crosses  gave 
pure  parental  types  as  well  as  other  lint  lengths  which  appeared 


COTTON  AND  SORGHUM  177 

homozygous.  Correlation  between  length  of  lint  and  size  of 
seed  may  explain  some  complications. 

Mutations  in  Cotton. — The  cotton  plant,  like  (Enothera,  has 
often  been  spoken  of  as  having  germinal  instability  and  likely 
to  produce  mutations.  While  mutations  undoubtedly  do  occur, 
it  is  likewise  highly  probable  that  many  of  the  so-called  mutations 
are  simply  segregates  of  a  former  natural  cross.  The  ease  with 
which  natural  crossing  occurs  and  the  large  number  of  chromo- 
somes (20  according  to  Balls)  contained  in  the  cotton  gamete 
facilitate  the  appearance  of  forms  differing  from  the  general  type. 
The  larger  the  number  of  haploid  chromosomes  the  more  difficult 
it  is  to  secure  homozygous  individuals  after  a  cross.  Egyptian 
cotton  is  described  by  Kearney  (1914)  as  being  a  mutating  type. 
From  it  the  varieties  Yuma,  Pima,  and  Gila  are  supposed  to 
have  arisen.  In  this  connection  it  is  of  interest  to  point  out  that 
the  common  belief  as  to  the  origin  of  Egyptian  cotton  is  that  it 
arose  by  hybridization  between  a  brown-linted  tree  cotton  and 
American  sea  island.  The  subsequent  development  is  unknown. 

In  view  of  the  foregoing  and  the  fact  that  no  convincing  evi- 
dence has  been  presented  to  the  contrary,  the  present  writers 
believe  that  many  of  these  supposed  mutations  are  in  reality 
factorial  recombinations  resulting  from  natural  crossing. 

Cotton  Breeding. — Cotton  improvement  by  breeding  may  be 
sought  along  lines  similar  to  those  followed  with  all  naturally 
selfed  crops.  In  producing  pure-line  material  for  scientific 
study  and  subsequent  hybridization  it  is  essential  to  obtain  ab- 
solute self-pollination. 

From  a  commercial  standpoint,  a  productive  cotton  with  long 
lint  and  smooth  seed  is  desirable.  Webber  (1905)  crossed 
Klondike,  a  productive  upland  variety,  and  sea  island,  which  has 
long  lint  and  smooth  seed.  Out  of  an  F2  generation  consisting 
of  several  thousand  plants,  only  12  combined  the  large  blunt  bolls 
of  the  upland  with  the  long  lint  and  black  seed  of  sea  island. 
The  progeny  of  each  of  these  12  plants  was  grown  in  isolated 
plots  and  subjected  to  vigorous  selection.  In  the  fifth  generation 
a  number  of  plants  gave  progeny  " nearly  fixed  in  type." 

Resistance  to  wilt  disease  is  a  character  of  considerable  com- 
mercial importance.  This  disease  is  caused  by  Fusarium  vasin- 
fectum  Atk.  which  according  to  Orton  attacks  only  the  cotton  and 
its  near  relatives.  By  the  plant-to-row  method  and  under  wilt 
infection  conditions,  it  was  found  possible  to  build  up  varieties 
12 


178  BREEDING  CROP  PLANT* 

which  are  resistant  to  wilt.  A  number  of  resistant  high-yielding 
varieties  have  been  introduced  in  the  cotton  growing  regions  of 
the  United  States.  The  character  of  wilt  resistance  was  trans- 
mitted in  crosses  but  nearly  every  cross  gave  a  different  result. 
In  general,  resistance  proved  dominant  but  there  was  often  con- 
siderable variability,  possibly  due  to  the  gametic  composition 
of  the  parents  or  to  the  nature  of  the  reaction  between  the  disease 
organism  and  the  host  plant  or  to  the  lack  of  uniform  evironmental 
conditions.  Wilt  resistance  does  occur  and  varieties  may  be 
obtained  which  are  resistant  and  are  also  of  good  quality  with 
respect  to  yield  and  staple. 

SORGHUM 

Origin. — The  numerous  diversified  forms  of  sorghum  indicate 
that  it  has  been  cultivated  a  long  time.  Evidence  has  been  found 
that  it  was  grown  in  Egypt  as  early  as  2200  B.C.  Hackel  places 
all  the  cultivated  sorghums  and  the  various  forms  of  Johnson 
grass  in  one  botanical  species.  It  has  been  pointed  out  by  Piper 
(1916)  that  two  species  exist — the  perennials,  Johnson  grass  and 
its  varieties  (Andropogon  halepensis),  and  the  annual  sorghums 
(Andropogon  sorghum).  The  former  possesses  rootstocks,  and 
it  is  difficult  to  cross  it  with  either  the  cultivated  or  wild  forms  of 
sorghum. 

The  wild  annual  sorghums,  which  are  found  almost  exclusively 
in  Africa,  cross  readily  with  the  cultivated  forms.  Africa  is 
thought  to  be  the  native  home  of  our  cultivated  sorghums. 

Classification  and  Inheritance. — On  the  basis  of  the  three 
economic  characters — production  of  grain,  sugar,  and  broom- 
straw — three  distinct  types  of  sorghums  have  been  developed. 
All  of  these  produce  forage  and  some  of  them,  as  Sudan 
grass,  are  grown  primarily  for  this  purpose.  Piper,  after  Ball,  has 
suggested  a  group  classification  for  all  the  forms  of  A.  sorghum, 
cultivated  in  America,  to  which  the  student  is  referred  (Piper, 
1916).  Only  a  brief  statement  will  be  given  here.  Small- 
stemmed  sorghums,  such  as  Sudan-grass  and  Tunis-grass, 
comprise  one  group.  The  other  group,  the  large-stemmed  sor- 
ghums, are  divided  on  the  basis  of  the  character  of  the  pith — 
whether  it  is  juicy  or  dry.  The  juicy  sorghums  may  be  either 
sweet  or  slightly  sweet  to  sub-acid.  The  dry  sorghums  are  fur- 
ther classified  into  varieties  on  the  basis  of  panicle  char- 
acteristics. Hilson  (1916)  found  that  a  pithy  stalk  was  dominant 


COTTON  AND  SORGHUM  179 

to  a  sweet  stalk.  The  second  generation  of  the  cross  segre- 
gated as  a  monohybrid.  Graham  (1916)  of  India,  studied  the  in- 
heritance of  length  of  glume  and  color  of  seed-coats  in  some 
natural  and  artificial  crosses.  Long  and  short  glumes  behaved  as 
a  simple  Mendelian  pair  with  the  former  dominant.  In  the  in- 
heritance of  color  of  grain  a  series  of  multiple  allelomorphs  are 
involved.  Red  may  be  allelomorphic  to  yellow  or  white  and  like- 
wise yellow  may  be  allelomorphic  to  white.  The  usual  color 
dominance  is  shown.  Sometimes  when  yellow  and  white  are 
crossed  the  heterozygote  is  red  and  in  the  next  generation  segre- 
gates with  a  9  red  :  3  yellow  :  4  white  ratio.  Graham  suggests 
that  certain  of  the  white  seeds  are  undeveloped  reds  requiring  the 
presence  of  yellow  to  cause  the  development  of  the  red  color. 

Some  Results  of  Selection. — Sorghum  improvement  by  breed- 
ing has  been  accomplished  principally  through  selection.  Dwarf 
forms  have  occurred  in  most  varieties  and  have  furnished  material 
for  the  production  of  such  varieties  as  Dwarf  Milo,  Dwarf  Kafir, 
etc.  These  varieties  have  been  isolated  through  selection.  Sugar 
content  has  also  been  improved.  Failyer  and  Willard  conducted 
selection  experiments  at  the  Kansas  Station  from  1884  to  1903. 
During  that  time  they  increased  the  sugar  content  of  the  Orange 
variety  from  12.62  to  16. 10  per  cent.  At  the  Delaware  Agricultural 
Experiment  Station  even  more  striking  results  were  obtained 
(Neale,  1901).  The  variety  Amber,  from  which  selections  were 
made,  contained  on  the  average  11  per  cent,  sugar  with  a  purity 
of  65.  One  of  the  selections  made  from  it  had  a  sugar  content 
of  18.2  per  cent,  with  a  purity  of  81.  Dillman  (1916),  of  the 
United  States  Department  of  Agriculture,  made  several  selec- 
tions from  Minnesota  Amber  with  the  object  of  securing  an  early 
maturing,  drought  resistant  strain.  One  of  the  selections,  Dakota 
Amber,  has  proved  valuable.  It  is  more  dwarf  in  habit  of  growth 
than  Minnesota  Amber  and  matures  15  days  earlier.  It  produces 
excellent  forage  as  well  as  abundant  seed.  Early  dwarf  forms, 
as  a  rule,  are  more  drought  resistant  than  late  ones. 

Method  of  Breeding  Sorghum. — Sorghum  belongs  to  the 
naturally  self-fertilized  group  of  farm  crops  and  the  essential 
features  of  breeding  it  are  the  same  as  for  the  group.  However, 
sorghum  is  more  frequently  cross-fertilized  than  most  of  the 
other  naturally  selfed  crops  and  for  this  reason  it  is  necessary 
to  resort  to  bagging  the  panicles,  where  different  lines  are  grown 
in  close  proximity  to  one  another.  That  bagging  does  not  inhibit 


180  BREEDING  CROP  PLANTS 

the  setting  of  viable  seed  is  shown  by  the  work  of  Connor,  Ball, 
Ten  Eyck,  Townsend,  and  Leidigh,  all  of  whom  secured  viable 
seed  from  panicles  so  protected.  Leidigh  (1911)  credits  Connor 
with  the  statement  that,  "a  particular  strain  of  Orange  sorghum 
which  he  grew  two  generations  from  seed,  bagged  each  year, 
possessed  extraordinary  vitality  and  vigor  and  was  remarkably 
pure  and  uniform."  Townsend  (1909)  obtained  similar  results. 
From  the  foregoing  facts  it  is  evident  that  sorghums  should  be 
bred  as  a  self -fertilized  crop.  Bagging  the  panicles  is  a  necessary 
precaution  where  different  lines  are  grown  near  one  another. 
By  means  of  roguing  chance  mixtures  and  crosses  are  eliminated 
and  varieties  are  kept  in  a  pure  condition.  The  isolated  seed 
plot  also  is  recommended  as  a  correct  farming  practice. 


CHAPTER  XIV 
MAIZE  BREEDING 

Maize  was  the  most  important  bread  crop  of  the  American  In- 
dians and  even  today  is  the  most  important  crop  in  the  western 
hemisphere.  The  Indians  brought  the  culture  of  maize  to  a 
high  state  of  advancement  and  developed  innumerable  varieties. 
On  the  foundations  made  by  the  Indians  modern  corn-breeding 
has  made  marked  advances,  but  perhaps  no  North  American 
varieties  are  so  notable  as  those  developed  by  the  Incas  in  Peru. 

Origin  and  Species. — It  is  generally  believed  that  Mexico  is  the 
original  home  of  the  maize  plant,  although  there  is  no  absolute 
proof  of  this  (Harshberger,  1897).  Zea  mays  L.,  belongs  to  the 
tribe  Maydeae  of  the  order  Gramineae.  All  varieties  of  Indian 
corn  are  placed  in  the  species  mays.  The  nearest  relative  of 
maize  is  teosinte,  EucMcena  mexicana  Schrad.  Teosinte  and 
maize  cross  readily  and  a  natural  hybrid  between  these  cultivated 
grasses  was  described  under  the  name  Zea  canina  by  Watson 
(Harshberger,  1904).  A  study  of  these  crosses  led  Harshberger 
(1904,  1909)  to  make  the  hypothesis  that  maize  originated  from  a 
hybrid  between  a  sport  of  Euchlsena  and  normal  teosinte.  Mont- 
gomery (1906)  reached  the  conclusion  that  maize  and  teosinte 
had  a  common  progenitor.  It  was  considered  likely  that  the 
ancestral  form  of  these  cultivated  grasses  was  a  large  much- 
branched  grass  "each  branch  being  terminated  by  a  tassel-like 
structure  bearing  hermaphrodite  flowers."  As  evolution  pro- 
gressed, the  lateral  branches  of  maize  came  to  bear  only  pistillate 
flowers  and  the  central  branch  staminate  flowers.  This  theory 
is  strengthened  by  the  types  of  inflorescence  which  frequently 
appear  in  maize  varieties.  Often  the  central  spike  of  the  tassel  of 
lateral  branches  bears  seeds,  while  the  side  branches  of  the  same 
tassel  bear  only  staminate  organs.  All  gradations  appear 
between  the  normal  ear  of  maize  and  the  staminate  tassel.  It  is 
not  uncommon  in  self-fertilized  maize  races  to  obtain  plants  in 
which  the  tassel  of  the  main  branch  bears  both  male  and  female 

181 


182  BREEDING  CROP  PLANTS 

organs.  These  various  abnormalities  tend  to  support  the 
hypothesis  outlined  by  Montgomery. 

Collins  (1912)  has  supported  the  hypothesis  that  maize 
originated  as  a  hybrid  between  teosinte  and  an  unknown  grass 
belonging  to  the  tribe  Andropogonese.  This  grass  is  believed  to 
be  somewhat  like  some  varieties  of  pod  corn  (Zea  mays  tunicata) 
which  produce  seeds  only  in  the  tassel  and  are  in  many  essen- 
tial characters  strongly  contrasted  with  teosinte.  These  conclu- 
sions have  been  reached  after  extensive  studies  of  many  primitive 
varieties  of  maize,  teosinte,  and  hybrids  between  teosinte  and 
maize.  Collins  especially  emphasizes  the  fact  that ' '  in  practically 
every  case  where  there  is  pronounced  divergence  between  teosinte 
and  pod  corn,  maize  shows  characters  of  an  intermediate  nature 
and  these  characters  are  usually  variable." 

Kuwada  (1919,  abstract  by  Ikeno,  1920)  has  published  cyto- 
logical  support  for  this  theory.  He  finds  the  chromosomes  of 
maize  to  be  of  two  types,  long  and  short.  He  also  finds  that 
Euchlaena  has  10  haploid  chromosomes  which  are  long,  and 
Andropogon  likewise  has  the  same  number  of  haploid  chromo- 
somes which  are  distinguished  by  their  shortness. 

Sturtevant  (1899)  divided  the  species  Zea  mays  into  several 
groups  and  considered  each  of  specific  rank.  The  more  common 
practice  is  to  make  the  five  major  groups  sub-species,  retaining 
the  monotypic  species  Zea  mays.  This  plan  was  followed  by 
East  (see  East  and  Hayes,  1911).  A  short  description  of  the 
differential  characters  of  these  five  groups  is  given  here. 

Zea  mays  tunicata,  the  pod  corns,  Sturtevant,  Bulletin  Torrey 
Botanical  Club,  1894,  page  355. 

"In  this  group  each  kernel  is  enclosed  in  a  pod  or  husks,  and 
the  ear  thus  formed  is  enclosed  in  husks."  This  is  perhaps  the 
least  deserving  of  sub-specific  rank  as  it  is  an  unfixable  group 
(seepage  189). 

Zea  mays  indurata,  the  flint  corns,  Sturtevant,  Bulletin  Torrey 
Botanical  Club,  1894,  page  355. 

The  group  comprises  those  varieties  with  a  starchy  endosperm 
in  which  the  soft  starch  is  surrounded  by  corneous  starch.  The 
proportions  of  soft  and  corneous  starch  vary  considerably  in 
different  varieties. 

Zea  mays  everta,  the  pop  corns.  Sturtevant,  Bulletin  Torrey 
Botanical  Club,  1894,  page  325. 

In  this  group  there  is  only  a  small  proportion  of  soft  starch 


MAIZE  BREEDING  183 

in  the  endosperm  and  a  correspondingly  large  proportion  of 
corneous  starch.  Some  seeds  may  be  entirely  free  from  soft 
starch,  but  there  is  generally  some  soft  starch  surrounding  the 
germ.  The  group  is  characterized  by  the  small  size  of  its  seeds 
and  ears. 

Zea  mays  indentata,  the  dent  corns.  Sturtevant,  Bulletin 
Torrey  Botanical  Club,  1894,  page  329. 

The  corneous  starch  in  this  group  is  located  at  the  sides  of  the 
seed  and  the  soft  starch  extends  to  the  summit.  The  soft 
starch  dries  more  rapidly  than  the  corneous  and  this  produces 
the  shrinkage  which  causes  the  characteristic  indentation  of  the 
seed. 

Zea  mays  amylacea,  the  soft  or  flour  corns,  Sturtevant,  Bulletin 
Torrey  Botanical  Club,  1894,  page  331. 

This  group  is  recognized  by  an  almost  entire  absence  of  cor- 
neous starch.  There  is  no  indentation  in  some  varieties  and  only 
a  slight  one  in  others.  The  soft  starch  content  characterizes 
this  group. 

Zea  mays  saccharata,  the  sweet  corns.  Sturtevant,  Bulletin 
Torrey  Botanical  Club,  1894,  page  333. 

"A  well-defined  species  group  characterized  by  the  trans- 
lucent, horny  appearance  of  the  kernels  and  their  more  or  less 
crinkled,  wrinkled,  or  shriveled  condition."  East  (1910d)  pre- 
sented evidence  which  shows  that  the  sweet  corns  are  dent,  flint, 
or  pop  varieties  which  have  not  the  ability  to  mature  starch  nor- 
mally. The  few  starch  grains  produced  are  small,  angular,  and 
imperfect. 

INHERITANCE  OF  CHARACTERS 

Endosperm  Characters. — The  word  xenia  was  first  used  by 
Focke  (1881)  to  denote  the  effect  which  was  apparently  produced 
by  the  action  of  pollen  upon  the  maternal  tissue  of  the  seed. 
The  endosperm  of  maize  was  cited  as  a  classical  example  of  such 
an  effect.  After  the  discovery  by  Guignard  (1899)  and  Nawas- 
chin  (1898)  that  the  polar  nuclei  of  the  endosperm  fuse  with  the 
second  male  nucleus  of  the  pollen  grain,  De  Vries  (1899),  Correns 
(1899),  Webber  (1900),  and  Guignard  (1899,  1901)  saw  that  this 
furnished  an  explanation  of  xenia  in  maize.  From  a  considera- 
tion of  inheritance  of  endosperm  character  the  following  law  of 
xenia  may  be  formulated : 


184  BREEDING  CROP  PLANTS 

Xenia  may  result  from  crossing  varieties  which  differ  in  a 
single  visible  endosperm  character.  When  a  character  difference 
is  dependent  on  a  single  dominant  factor,  xenia  occurs  only  when 
the  factor  is  carried  by  the  male  parent,  or,  when  dominance  is 
incomplete,  xenia  results  when  either  variety  is  the  male.  When 
a  character  difference  is  dependent  on  more  than  one  factor,  all 
located  in  one  parent,  and  dominance  appears  complete,  xenia 
occurs  only  when  these  differential  factors  are  located  in  the 
male;  when  dominance  is  incomplete,  xenia  occurs  if  the  factors 
are  located  in  either  parent.  When  two  varieties  have  a  similar 
character  or  a  different  character  expression  but  contain  between 
them  endosperm  factors  necessary  for  the  production  of  a  new 
character,  xenia  occurs  when  either  variety  is  the  male. 

The  inheritance  of  an  intermediate  starchy-sweet  (called 
pseudo-starchy)  condition,  which  is  often  present  in  some  sweet 
corn  ears,  has  been  studied  by  Jones  (1919).  Three  factors  were 
shown  to  explain  the  results:  (1)  a  plant  factor,  A,  necessary  for 
complete  expression  of  the  so-called  pseudo-starchy  character; 
(2)  an  endosperm  factor,  B,  which  prevents  the  characteristic 
shrinking  of  sweet  seeds;  (3)  an  endosperm  factor,  C,  determining 
opaqueness.  C  gives  complete  dominance,  while  A  and  B  give  an 
intermediate  condition  when  heterozygous,  and  B  in  addition 
shows  a  cumulative  effect  in  proportion  to  the  number  of  factors 
involved.  C  and  c  give  the  greatest  differential  effect  only 
in  the  presence  of  the  homozygous  condition  for  A  and  B.  From 
this  brief  discussion  it  is  easy  to  see  that  reciprocal  crosses 
between  AABBcc  X  aabbCC  will  not  give  like  results.  AABBcc 
fertilized  with  aabbCC  will  give  an  endosperm  condition  ABBbcC, 
while  the  reciprocal  cross  will  give  abbBCc.  As  A  is  necessary 
for  recognizable  expression  of  pseudo-starchiness,  one  cross 
will  show  xenia  while  its  reciprocal  will  not. 

The  following  endosperm  characters  have  been  studied  and 
the  results  are  briefly  summarized.  (See  Table  XXXIX.) 

The  cross  between  the  waxy  variety  of  Chinese  maize  and 
American  sweet  varieties  is  of  interest,  as  in  FI  maize  with  a 
corneous  endosperm  was  obtained,  while  in  F%  a  ratio  of  9 
horny  to  4  sweet  to  3  waxy  seeds  was  obtained.  Many  starchj^- 
sweet  crosses  have  been  studied  and  as  yet  no  case  has  been 
obtained  which  showed  more  than  a  single  main  factor  difference. 
Apparently  the  sub-species,  Z.  mays  saccharata,  differs  by  only 
a  single  main  factor  from  the  starchy  subspecies. 


MAIZE  BREEDING 


185 


TABLES  XXXIX. — SUMMARY  OF  INHERITANCE  OF  ENDOSPERM  CHARACTERS 

OF  MAIZE 


Parents 

Fi 

F2 

Authority 

Chinese    Maize 

Horny 

Ratio  of  9  horny  to  4 

Collins  and  Kempton 

(waxy)  X  Ameri- 

sweet to  3  waxy. 

(1911)  (1914). 

can  Maize  (sweet). 

Starchy  (flint,  dent, 

Starchy 

3  starchy  to  1  sweet. 

Correns,     1901.     East 

or  pop)  X  Sweet. 

and  Hayes,  1911. 

Yellow    X    Colorless 

Completely  dominant 

3  yellow  to  1  white. 

Correns,  1901. 

endosperm. 

or  intermediate  yel- 

15 yellow  to  1  white. 

East  and  Hayes,  1911. 

low.    Absence  of  col- 

3 white  to  1  yellow. 

White,  1917. 

or  in  cross  studied  by 

White. 

Aleurone  color:  pur- 

Dominance or  partial 

Ratios  indicate  from 

East  and  Hayes,  1911. 

ple,  red,  or  white. 

dominance    of    color 

1    to    5    differential 

East,  19126.     Emer- 

usually.    Sometimes 

factors. 

son,  1912o.  Emerson, 

dominance  of  color- 

1918. 

less. 

Flour  Maize  X  Flint 

No  immediate  effect. 

Ratio  1  flour  seed  to 

Hayes  and  East,  1915. 

1  flint  seed  on  each 

Fi  ear. 

Flour  X  Pop  

No  immediate  effect. 

Segregation,  but  more 

Hayes  and  East,  1915. 

complex  than  in  flint- 

flour  cross. 

Normal  X  Defective 

Normal 

Segregation  on  a  3  :  1 

Jones,  1920. 

Seeds. 

or      more      complex 

ratio. 

Reciprocal  crosses  between  flour  and  flint  showed  no  immediate 
effect  of  cross-pollination.  The  ears,  however,  of  the  FI  plants 
showed  a  distinct  segregation  into  flour  and  flint  seeds  in  a  1  : 1 
ratio.  Later  generations  showed  that  the  results  were  most 
easily  explained  on  the  cumulative  factor  basis.  If  a  soft  flour 
variety  was  pollinated  by  a  flint  race,  the  endosperm  would  con- 
tain two  factors  for  soft  flour,  SS,  and  one  for  flint  condition, 
F,  or  SSF.  The  reciprocal  cross  would  be  FFS.  If  two  factors, 
FF  or  SS,  are  completely  dominant  over  one  factor,  S  or  F, 
respectively,  there  would  be  no  immediate,  effect  of  cross-polli- 
nation and  the  segregation  on  FI  ears  would  be  in  a  1:1  ratio. 
Dents  crossed  with  flour  races  give  a  very  similar  result,  but  the 
seeds  are  not  so  easily  distinguished  by  inspection.  Reciprocal 
crosses  between  pop  and  flour  races  show  no  immediate  effect  of 
pollination  with  complex  segregation  on  the  ears  of  FI  plants. 
Pure  flour  and  pop  forms  may  be  obtained  in  later  generations, 
but  the  results  cannot  be  explained  by  a  single  factor  difference. 
With  the  hypothesis  that  pop  and  flour  corns  differ  by  two  or 
more  main  factors  and  with  each  factor  behaving  in  a  some- 


186 


BREEDING  CROP  PLANTS 


what  similar  manner  as  in  the  flint-flour  cross,  the  difficulty  of 
a  correct  classification  by  inspection  is  apparent. 

The  endosperm  of  corn  may  be  either  yellow,  pale  yellow, 
or  white.  In  some  crosses  there  is  almost  complete  dominance  of 
the  yellow  color,  while  in  other  crosses  the  F\  is  intermediate  or 
pale  yellow.  The  results  of  most  yellow-white  crosses  may  be 
explained  by  one  factor  or  by  two  multiple  factors.  It  is  impos- 
sible to  tell  by  inspection  whether  a  particular  yellow  variety  con- 
tains one  or  two  factors  for  yellow.  The  only  sure  method  is 


FIG.  43. — Two  first  year  self-fertilized  ears  of  Minn.  No.  23  showing  the  lethal 
endosperm  character. 

to  note  whether  the  segregation  approaches  3:1  or  15:1.  White 
(1917)  has  recorded  a  cross  in  pop  corns  between  yellow  and 
white  endosperm  varieties  in  which  white  is  the  dominant  char- 
acter. The  results  were  explained  by  supposing  that  the  white 
variety  carried  an  inhibitory  factor,  A,  and  also  a  factor  for 
yellow  or  F,  while  the  zygotic  condition  of  the  yellow  variety 
was  YY. 

The  inheritance  of  aleurone  color  is  even  more  complex  than 
the  inheritance  of  yellow  endosperm  color.  The  aleurone  may 
be  either  colorless,  mottled,  red,  or  purple.  Three  factors  are 


MAIZE  BREEDING  187 

necessary  for  the  production  of  red  aleurone.  These  Emerson 
(1918)  has  called  R,  C,  and  A.  In  addition  to  these  three 
factors,  Pr,  in  either  the  simplex  or  duplex  condition,  gives 
purple  aleurone.  An  inhibitory  factor  which  was  called  /  was 
first  discovered  by  East  and  Hayes  (1911).  When  this  is  present, 
the  aleurone  layer  is  colorless.  Races  of  white  corn  exist  which 
contain  some  but  not  all  of  the  factors  necessary  for  the  produc- 
tion of  aleurone  color.  Certain  crosses  between  white  races 
give  colored  aleurone.  With  five  or  six  factors  involved,  it 
becomes  apparent  that  segregation  in  certain  cases  may  be  in  a 
simple  3:1  ratio,  while  segregation  in  other  crosses  may  give 
extremely  complex  ratios.  There  are  various  intensities  of  the 
purple  color  in  different  races.  These  have  been  discussed  in 
detail  by  Emerson  (1918).  Over  waxy  or  floury  endosperm 
purple  aleurone  gives  a  dull  black  appearance.  With  a  varia- 
tion in  color  of  the  endosperm  from  white  to  dark  yellow  there 
is  a  corresponding  variation  in  color  of  the  aleurone  from  purple 
to  brownish  shades.  These  differences  in  aleurone  appearance 
are  due  to  the  inheritance  of  other  genetic  factors  for  endosperm 
characters  beside  those  which  govern  the  ability  to  produce 
aleurone  color.  There  are  some  genetic  differences  in  aleurone 
colors  which  are  not  related  to  the  underlying  endosperm 
characters.  Two  color  patterns  have  been  mentioned  by 
Emerson  under  the  names  speckled  and  dark-capped.  The 
color  is  found  on  the  crown  of  the  seed  and  varies  from  a  mere 
speck  to  a  large  spot.  Both  color  patterns  are  recessive  to 
normal  or  self-color.  Aside  from  these  color  patterns  which  are 
apparent  in  homozygous  races,  there  are  mottled  colors  which 
are  only  obtained  in  the  heterozygous  condition.  Emerson  has 
given  quite  conclusive  proof  that  mottling  is  associated  with  the 
Rr  factor  pair.  Apparently  endosperms  of  the  constitution 
RRR  or  RRr  are  self-colored  while  Rrr  shows  mottling. 

PLANT  CHARACTERS 

Colors  in  Plant  Organs. — There  is  a  group  of  anthocyan  color 
characters  which  are  expressed  in  one  or  all  of  the  following 
organs:  cob,  pericarp,  silk,  tassel,  i.e.,  glume,  and  in  the  leaves 
and  stems.  There  are  several  different  character  expressions 
of  a  stable  nature  for  this  group  of  color  characters.  In  some 
cases  the  color  in  two  or  more  organs  may  be  inherited  as  if  due 


188 


BREEDING  CROP  PLANTS 


to  a  single  factor.  For  example,  the  color  in  cob  and  pericarp 
is  often  correlated  in  inheritance.  Emerson  (1911)  has  found  a 
case  in  which  the  factor  for  color  in  the  cob  behaves  as  an  allelo- 
morph of  the  factor  for  color  in  the  pericarp.  In  the  illustration 
given  in  Table  XL  Ri  represents  the  factor  for  cob  color  and  Rz 
the  factor  for  pericarp  color. 

TABLE  XL. — SUMMARY  OF  A  CROSS  IN  WHICH  A  FACTOR  FOR  COB  COLOR 
BEHAVED  AS  AN  ALLELOMORPH  OF  A  FACTOR  FOR  COLOR  OF  PERICARP 


Parents 

F, 

Zygotes 

Appearance 

Zygote 

Gametes 

Appearance 

Zygote 

Gametes 

Appearance 

Cob 

Peri- 
carp 

RiRi 

Ri 

Red  in  cob 

RiRt 

R\  or  Rt 

Red  cob,  red 

1  RiRi 

Red 

White 

**, 

R-- 

Red  in  peri- 
carp 

pericarp 

1    #2/?2 

Red 

White 

Red 
Red 

East  and  Hayes  (1911)  have  given  a  case  of  a  cross  between 
two  reddish  blush  pericarp  colors  which  developed  only  under 
light  conditions,  which  gave  a  15  : 1  ratio  in  F2.  This  indicates 
two  separately  inherited  factors. 

There  are  numerous  expressions  of  colors.  Hayes  (1917)  ob- 
tained four  pericarp  colors  which  bred  comparatively  true  when 
self -fertilized.  These  were  called  solid  red,  in  which  the  pericarp 
was  uniformly  red;  variegated,  in  which  the  color  was  in  deep 
red  stripes  of  various  sizes;  pattern,  in  which  the  color  was  also 
in  stripes  but  was  much  lighter  in  intensity;  colorless,  lacking 
color  in  the  pericarp.  The  factors  for  red,  variegated,  pattern 
and  colorless  appeared  to  form  a  series  of  multiple  allelomorphs. 
The  cross  between  pattern  and  variegated  gave  an  increase  in 
bud  sports  in  F\,  i.e.,  ears  which  produced  two  sorts  of  pericarp 
color  sharply  differentiated ;  while  in  F2  a  few  solid  red  ears  were 
obtained  and  many  striped  ears.  This  was  presented  as  an 
instance  in  heterozygous  material  in  which  a  change  in  a  charac- 
ter occurred.  Without  attempting  an  explanation  it  was  pointed 
out  that  no  such  change  occurred  in  six  generations  of  selection 
in  self-fertilized  families  of  the  red,  striped,  or  pattern  lines. 
Emerson  (1914a,  1917)  has  studied  the  inheritance  of  these 
anthocyan  colors  for  several  years.  To  explain  the  production 
of  solid  red  in  variegated  races,  he  supposes  a  change  or  mutation 


MAIZE  BREEDING  189 

in  the  factor  F,  for  variegated,  to  S  for  self-color.     Emerson 
concluded : 

"That  these  results  favor  the  idea  that  single  allelomorphic  factors, 
rather  than  two  or  more  closely  linked  factors,  are  responsible  for  the 
color  pattern  of  both  glumes  and  pericarp." 

The  concluding  paragraph  of  Emerson's  1917  paper  is  di- 
rectly in  line  with  the  ideas  which  have  been  developed  throughout 
this  book.  With  most  plant-breeding  material  of  our  farm 
crops,  there  is  no  evidence  for  basing  a  system  of  plant  improve- 
ment upon  mutations,  as  these  are  infrequent.  With  anthocyan 
color  characters  of  corn,  inherited  changes  sometimes  occur 
more  frequently  and  such  mutations  become  of  selection  value. 
This  does  not  invalidate  the  pure-line  conception  for  the  large 
number  of  cases  where  factor  stability  is  the  rule.  To  quote 
from  Emerson: 

"The  existence  of  the  series  of  at  least  nine  or  ten  multiple  allelo- 
morphs to  which  variegation  belongs,  indicates  that  a  factor  for  peri- 
carp color  has  mutated  several  times.  Some  of  the  factors  for  this  series 
have  not  been  observed  to  mutate,  while  others  have  mutated  rarely 
and  still  others  many  times.  In  fact,  the  principal  difference  between 
certain  of  the  factors  is  thought  to  lie  in  their  relative  frequencies  of 
mutation." 

Podded  Condition. — The  podded  character  was  thought  by 
East  and  Hayes  (1911)  to  be  a  simple  dominant  and  to  be  de- 
pendent on  a  single  factor  for  its  development.  Extracted 
recessives  bred  true  to  the  podless  condition.  Collins  (1917) 
has  presented  evidence  which  indicates  that  the  ordinary  type 
of  tunicate  maize  represents  a  case  of  imperfect  dominance  and 
that  it,  like  the  Andalusian  fowls,  is  unfixable  and  related  to  the 
heterozygous  condition.  Selfed  seeds  of  typical  podded  ears  pro- 
duced three  types  of  plants:  (1)  like  the  parent;  (2)  with  normal 
ears;  (3)  a  plant  which  does  not  produce  seed  in  the  lateral  in- 
florescences but  in  perfect  flowers  in  the  tassels.  Jones  and 
Gallastegui  (1919)  obtained  similar  results.  A  starchy  tunicate 
ear  was  used  as  the  female  parent  and  was  pollinated  with  pollen 
from  a  non-tunicate  sweet  race.  The  linkage  between  the 
starchy  and  tunicate  factors  was  quite  close,  only  8.3  per  cent, 
of  crossing-over  occurring. 

Auricle  and  Ligule. — Emerson  has  shown  that  the  absence  of 
auricle  and  ligule  is  a  recessive  character.  In  a  cross  between  a 


190  BREEDING  CROP  PLANTS 

pure  race  for  the  absence  of  these  characters  and  a  normal  variety 
all  FI  plants  had  normal  leaves.  In  F2,  ratios  of  672  normal- 
leaved  plants  to  221  liguleless  were  obtained. 

Chlorophyll  Inheritance. — Numerous  abnormalities  for  chlo- 
rophyll development  have  been  observed  in  corn,  many  of  which 
behave  as  simple  recessives  giving  a  3: 1  ratio  in  the  F2  of  a  cross 
between  the  normal  and  abnormal  form. 

Lindstrom  (1918)  has  reviewed  earlier  investigations  of 
chlorophyll  inheritance  and  has  made  a  careful  genetic  study 
of  several  different  chlorophyll  abnormalities.  Three  of  these — 
white,  virescent-white,  and  yellow — appear  in  the  seedling  stage. 
The  white  form  is  a  true  albino,  apparently  lacking  chloroplasts. 
The  virescent-white  appears  white  at  first,  but  under  favorable 
conditions  it  gradually  becomes  a  yellowish  green  color,  especially 
at  the  tips  of  the  leaves.  There  is  considerable  variation  in  the 
appearance  of  different  seedlings  of  this  type  but  genetically  all 
behave  alike.  The  yellow  type  gives  seedlings  with  a  yellow 
color.  Both  the  white  and  the  yellow  seedlings  die  before 
maturity. 

The  normal  green  form  behaves  as  an  allelomorph  to  the  various 
seedling  abnormalities  and  contains  the  three  dominant  genes, 
W,  V,  and  L.  Counts  of  the  number  of  normal  green  plants 
and  the  three  seedling  types  obtained  from  various  heterozygous 
plants  are  as  follows: 

Green 1,513     White 555 

Green 4,297     Virescent-white 1,394 

Green 1,493     Yellow 532 

Virescent-whites  which  turned  green  on  maturity  were  selfed 
and  produced  a  progeny  consisting  of  717  virescent-white  seed- 
lings and  nine  green.  The  latter  were  due  probably  to  stray 
pollen. 

From  a  study  of  interrelation  of  these  various  factors,  Lind- 
strom has  concluded  that  the  following  phenotypic  formulas  ex- 
plain the  appearance  of  different  sorts  of  seedlings; 

GREEN  VIRESCENT-WHITE  YELLOW  WHITE 

LVW  LvW  IvW  LVw 

IVW  Lvw 

IVw 
Ivw 

These    studies    have    considerable    bearing    on    the    present 


MAIZE  BREEDING  191 

conception  of  inbreeding  and  cross-breeding  as  applied  to  corn 
improvement.  Lindstrom  found,  for  example,  that  plants  con- 
taining the  wW  combination  were  less  vigorous  than  WW 
forms.  As  a  rule,  a  Ww  plant  produced  only  a  single  stalk  which 
was  easily  blown  over  in  a  strong  wind. 

There  are  also  abnormal  chlorophyll  types  which  appear  in 
the  mature  plant.  Of  these,  golden,  green-striped,  fine-striped, 
and  japonica  types  are  simple  Mendelian  recessives  to  normal 
green.  In  the  golden  type,  when  a  month  or  more  old,  the  green 
color  begins  to  disappear.  The  golden  type  is  not  very  vigorous 
toward  maturity.  It  produces  abundant  pollen  and  small  ears. 
The  green-striped  form  appears  about  two  months  after  germina- 
tion. These  stripes  are  uniform  in  distribution,  green  and  lighter 
areas  alternating,  and  running  parallel  through  the  leaf.  Mature 
green  striped  plants  are  less  vigorous  than  normal  green  forms  and 
the  leaves  wilt  more  severely  on  hot  days.  The  japonica  types  are 
striped  with  green,  pale  yellow,  yellow,  and  white,  and  are  well 
known,  being  frequently  used  for  ornamental  planting.  These 
forms  are  more  vigorous  than  the  golden  or  green-striped  types. 
There  are-  also  fine-striped  and  spotted  forms.  The  spotted 
forms  have  not  as  yet  been  studied  thoroughly. 

Four  of %  the  mature  plant  chlorophyll  types  have  been  found 
to  be  recessive  to  the  normal  green  forms.  The  following 
genetic  factors  have  been  used  by  Lindstrom: 

g  —  golden  type 
st  —  green-striped 
j  —  japonica 
/  —  fine-striped 

The  following  summary  expresses  the  factorial  condition  of 
these  forms  of  chlorophyll  abnormality; 

CHLOROPHYLL  TYPES  CHLOROPHYLL  FACTORS 

Green WVLGStJF  or  WVlGStJF 

White wVLGStJF 

Virescent-white WvLGStJF 

Yellow WvlGStJF 

Golden WVLgStJF  or  W VlgSUF 

Green-striped WVLGstJF 

Japonica  white-striped WVLGStjF 

Japonica  yellow-striped WVlGStjF 

Fine-striped WVLGStJf 


192  BREEDING  CROP  PLANTS 

Studies  of  the  linkage  relations  of  these  chlorophyll  factors 
have  been  made.  The  seedling  factors  w  and  v,  and  v  and  I 
show  independent  inheritance.  The  factors  which  influence  the 
chlorophyll  development  in  the  mature  plant,  g  and  st,  g  and 
j,  g  and  /,  j  and  st,  j  and  /,  appear  to  be  inherited  independently. 
Also  st  and  v  are  inherited  independently. 

The  linkage  relations  suggest  that  one  pair  of  chromosomes 
in  maize  contains  the  factor  pairs  Gg  and  LI  as  well  as  the  aleurone 
factors,  Rr.  The  japonica  striping  is  influenced  by  the  aleurone 
factor  R,  as  the  presence  of  R  represses  striping,  while  r  allows 
full  expression  of  the  pattern.  These  abnormalities  have  been 
discussed  in  some  detail  as  they  show  typical  Mendelian  in- 
heritance of  chlorophyll  characters  and  have  considerable 
bearing  on  the  improvement  of  corn  by  the  isolation  of  pure 
biotypes. 

Some  Seed  and  Ear  Characters. — Crosses  between  dents  and 
flints  were  studied  by  East  and  Hayes  (1911).  There  is  no 
immediate  visible  effect  of  foreign  pollen  on  the  endosperm  seed 
characters  which  separate  these  subspecies.  Segregation  occurred 
in  F2;  some  forms  were  obtained  in  F3  which  bred  true  to  flint 
habit;  some  bred  true  to  the  dent  type;  while  still  others 
showed  segregation.  Two  or  more  factors  were  necessary  to 
explain  results.  The  inheritance  of  the  pointed  condition  of 
the  seed  which  is  characteristic  of  white  rice  pop  was  also  studied 
by  Hayes  and  East  (1915).  It  was  found  possible  to  transfer 
this  pointed  condition  to  the  dent  subspecies.  Results  were 
complex  and  indicated  that  two  or  more  cumulative  factors 
were  involved. 

Size  Characters. — Emerson  and  East  (1913)  summarized 
inheritance  of  size  characters  of  seeds  and  ears.  Weight  of 
seed,  seed  measurement,  number. of  rows,  and  length  and  diame- 
ter of  ear  were  characters  studied.  In  general,  the  FI  condition 
was  intermediate,  and  complex  segregation  occurred  in  F^. 
The  inheritance  of  height  of  plant,  of  period  of  maturity,  and  of 
suckering  habit,  was  also  studied.  The  fact  that  a  considerable 
series  of  fairly  stable  varieties  is  known  which  exhibit  numerous 
conditions  of  the  development  of  particular  size  characters,  is  also 
evidence  of  a  complex  inheritance.  Segregation  occurred  in 
F2  and  extracted  forms  were  obtained  which  approached  the 
original  parental  conditions.  Intermediates,  as  well  as  extremes, 
sometimes  bred  true. 


MAIZE  BREEDING 


193 


Chemical  Composition. — The  classical  selection  experiments 
of  the  University  of  Illinois  for  the  purpose  of  isolating  high  and 
low  protein,  and  high  and  low  oil  strains,  are  well  known.  They 
prove  conclusively  that  strains  differing  in  chemical  composition 
may  be  isolated  by  selection.  Table  XLI  gives  the  results  for 
15  years'  selection.  This  information  was  obtained  through  the 
kindness  of  Professor  L.  H.  Smith. 

Progress  during  the  latter  years  of  the  experiment  has  not  been 
so  rapid  as  during  the  early  years,  which  is  probably  because  the 
genetic  limit  for  high  and  low  protein  and  high  and  low  oil  pro- 

TABLE  XLI. — A.  RESULTS  OF  SELECTING  MAIZE  FOR  HIGH  AND  FOR  Low 

PROTEIN  CONTENT  RESPECTIVELY 
Average  Percentage  Protein  in  Crop  Each  Generation 


Year 

High 
strain 

Average 
for  period 

Low 

strain 

Average 
for  period 

Difference 

Difference 
for  period 

1896 

10.92 

10.92 

1897 

11.10 

10.55 

0.55 

1898 

11.05 

10.55 

0.50 

1899 

11.46 

9.86 

1.60 

1900 

12.32 

11.37 

9.34 

10.24 

2.98 

1.12 

1901 

14.12 

10.04 

4.08 

1902 

12.34 

8.22 

4.12 

1903 

13.04 

8.62 

4.42 

1904 

15.03 

9.27 

5.76 

1905 

14.72 

13.85 

8.57 

8.94 

6.15 

4.91 

1906 

14.26 

8.64 

5.62 

1907 

13.89 

7.32 

..... 

6.57 

1908 

13.94 

8.96 

4.98 

1909 

13.41 

7.65 

5.76 

1910 

14.87 

14.07 

8.25 

8.16 

6.62 

5.91 

1911 

13.78 

7.89 

5.89 

1912 

14.48 

8.15 

6.33 

1913 

14.83 

7.71 

7.12 

1914 

15.04 

7.68 

7.36 

1915 

14.53 

14.53 

7.26 

7.74 

7.27 

6.79 

1916 

15.66 

8.68 

6.98 

1917 

14.44 

7.08 

7.36 

1918 

15.48 

7.13 



8.35 

1919 

14.70 

6.46 

8.24 

13 


194 


BREEDING  CROP  PLANTS 


B.  RESULT  OF  SELECTING  MAIZE  FOR  HIGH  AND  FOR  Low  OIL  CONTENT 

RESPECTIVELY 
Average  Percentage  Oil  in  Crop  Each  Generation 


Year 

High 
strain 

Average               Low 
for  period            strain 

Average 
for  period 

Difference 

Difference 
for  period 

1896 

4.70 

4.70 

1897 

4.73 

4.06 

0.67 

1898 

5.15 

3.99 

1.16 

1899 

5.64             ....             3.82 

1.82 

1900 

6.12             5.41             3.57 

3.86 

2.55 

1.24 

1901 

6.09 

3.43 

2.66 

1902 

6.41 

3.02 

3.39 

1903 

6.50 

2.97 

.... 

3.53 

1904 

6.97 

2.89 

.... 

4.08 

1905 

7.29 

6.65             2.58 

2.98 

4.71 

3.67 

1906 

7.37 

2.66 

4.71 

1907 

7.43 

....             2.59 

.... 

4.84 

1908 

7.19 

2.39 

4.80 

1909 

7.05 

2.35 

4.70 

1910 

7.72 

7.35             2.11 

2.42 

5.61 

4.93 

1911 

7.51 

2.05 

5.46 

1912 

7.70 

2.18 

5.52 

1913 

8.15 

1.90 

.... 

6.25 

1914 

8.29 

1  .  98 

6.31 

1915 

8.46 

8.02             2.07 

2.03 

6.39 

5.99 

1916 

8.50 

2.08 

6.42 

1917 

8.53 

2.09 

6.44 

1918 

9.35 

1.87 

7.48 

1919 

9.05 

1.77 

7.28 

duction  has  nearly  been  obtained.  These  new  strains  have  been 
named  Illinois  High  Protein,  Illinois  Low  Protein,  Illinois  High 
Oil,  and  Illinois  Low  Oil  respectively.  The  high  and  Low  Protein 
strains  were  crossed  with  a  normal  Learning  variety  by  Hayes 
(1913a).  The  FI  generation  of  the  cross  between  Low  Protein 
and  Learning  produced  approximately  the  same  protein  content 
as  Illinois  Low  Protein,  while  the  cross  between  Learning  and 
Illinois  High  Protein  gave  about  the  same  protein  content  in  FI 
as  the  normal  Learning  variety.  Results  are  given  in  Table  XLII. 


MAIZE  BREEDING 


195 


TABLE  XLTI. — INHERITANCE  OF  PROTEIN  IN  THE  FIRST  GENERATION  CROSSES 

BETWEEN  ILLINOIS  Low  PROTEIN  AND  ILLINOIS  HIGH  PROTEIN 

AND  STADTMUELLER'S  LEAMING 


Variety 

Number  of 
ears  analyzed 

Variation  in 
ears  in  protein 
content 

Average  pro- 
tein content, 
dry  basis 

Illinois  High  Protein 

19 

11    95-17    10 

14.87 

Learning,  1910  seed  

13 

7.75-16.28 

11.85 

FI  Cross       ....              ... 

12 

9.25-14.68 

11.85 

Illinois  Low  Protein  
Learning,  1911  seed 

16 
14 

6.81-11.56 
8  21-15  94 

9.41 
12  19 

FI  Cross  

9 

7.69-11.86 

9.18 

Self-fertilization  seems  a  logical  means  of  obtaining  pure  races 
of  different  chemical  compositions.  Numerous  ears  should  be 
self-fertilized  and  analyzed.  Those  that  appear  of  promise  may 
then  be  used  and  their  breeding  nature  determined  by  the  prog- 
eny test.  As  soon  as  homozygous  forms  containing  the 
desired  characters  have  been  isolated,  they  may  be  used  as 
foundation  stock  for  the  production  of  an  improved  variety. 
That  high  protein  races  may  thus  be  isolated  has  been  shown  by 
Hayes  and  Garber  (1919). 

TABLE  XLIII. — PROTEIN  CONTENT  OF  SELFED  STRAINS  OF  MINNESOTA  No. 
13  AND  CROSSES  BETWEEN  THEM 

Average  protein  content 


retrain  rso. 

1916 

1917 

1918 

1 

15  82 

14  03 

15  10 

4 

14  47 

13  06 

14  93 

Normal 

No.  13 

10  17 

10  25 

1  X  4  F 

!  Ear  4  

12.25 

1  X  4  F 

i  Ear  .B.  . 

12  44 

1  X  4F 

j  Ear  A:  

12.81 

These  same  workers  showed  that  there  was  a  correlation  be- 
tween the  number  of  seeds  produced  by  particular  self-fertilized 
FI  ears  of  the  crosses  A,  B,  and  K,  and  protein  content.  Low 
number  of  seeds  per  ear  was  correlated  with  high  protein  content. 
The  FI  crosses,  A,  5,  and  K,  yielded  slightly  more  than  normal 


196 


BREEDING  CROP  PLANTS 


corn  and  gave  2.5  per  cent,  higher  protein  content.  These 
particular  strains,  1  and  4,  were  not  examined  during  the  seedling 
stage  and  consequently  it  was  not  then  known  that  strain  1  was 
heterozygous  for  the  white  seedling  chlorophyll  abnormality 
which  Lindstrom  has  designated  by  the  factor  w.  In  the  second 
generation  grown  from  self-fertilized  ears  of  A,  B,  and  K,  ap- 
proximately one-fourth  of  the  seedlings  were  pure  white.  Lind- 
strom has  shown  that  plants  heterozygous  for  the  chlorophyll 
factors  Ww  are  slightly  less  vigorous  than  homozygous  green 
forms.  These  facts  lead  one  to  expect  that  high  protein  races 
with  good  yielding  ability  may  be  produced.  On  the  other  hand, 


FIG.  44. — Two  high  protein  strains  of  Minn.  No.  13  at  left  and  right  respectively 
which  have  been  self-fertilized  for  five  years  and  first  generation  cross  between 
them  in  the  center.  The  Fi  yielded  slightly  more  than  normal  Minn.  No.  13  and 
analyzed  2^  per  cent,  higher  in  protein  content. 

maximum  yield  of  grain  and  high  protein  content  probably  can 
not  be  obtained  in  the  same  variety. 

CORN  IMPROVEMENT  BY  THE  TRAINED  PLANT  BREEDER 

A  uniform  technic  has  been  developed  for  the  small-grain  breeder. 
With  corn,  however,  the  correct  method  of  breeding  is  even  yet 
somewhat  problematical.  Investigations  have  helped  to  clarify 
our  ideas  regarding  the  value  of  different  methods  of  wcrk. 
For  the  farmer  the  results  obtained  have  tended  to  simplify  pre- 
vious ear-to-row  methods.  For  the  technical  breeder,  how- 
ever, the  application  of  Mendelian  principles  has  resulted  in 
several  plans,  some  of  which  appear  rather  complex.  Their 


MAIZE  BREEDING 


197 


justification  apparently  rests  on  a  firm  genetic  foundation.  As 
yet  practical  demonstrations  of  improvement  in  corn  by  the 
application  of  Mendelian  principles  are  unavailable. 

Relation  of  Ear  Characters  to  Yield. — Corn  shows  have  accom- 
plished much  in  teaching  growers  the  characteristics  of  various 
standard  varieties.  They  have,  however,  over-emphasized  the 
value  of  ear  type  as  a  means  of  corn  improvement.  Much  work 
has  been  carried  on  with  the  view  of  determining  the  relation 
between  various  ear  and  plant  characters  and  ability  to  give 
high  yields.  In  general,  no  single  character  has  been  found  to  be 
so  closely  related  with  yielding  ability  as  to  be  of  much  value 
from  the  standpoint  of  selection.  Too  close  uniformity  of  type 
probably  tends  to  reduce  yield,  for  we  have  learned  that  self- 
fertilization  in  corn  causes  a  marked  decrease  in  growth  vigor  as 
compared  with  cross-fertilization. 

For  the  purpose  of  illustrating  the  general  nature  of  results 
in  this  field,  the  work  of  Williams  and  Welton  (1915),  in  Ohio, 
may  be  used.  They  compared  the  yields  of  ears  selected  on  the 
basis  of  wide  differences  of  type.  In  the  majority  of  cases 
selection  was  continuous,  i.e.,  long  ears  from  the  long  strain  and 
short  ears  from  the  short  strain.  Summarized  results  are  given 
in  Table  XLIV. 


TABLE  XLIV. — RELATION  BETWEEN  EAR  CHARACTERS  AND  YIELD 


Characters  worked  with 

Length 
of  test, 
years 

Differences  in  yield,  bu. 

Long  vs.  short  ears 

10 

Long         1  39 

Cylindrical  vs.  tapering.  ...            . 

9 

Tapering  1  .  65 

Bare  vs.  filled  tips.    . 

g 

Filled        0  34 

Rough  vs.  smooth  dent  
High  vs.  low  shelling  percentage  

7 
6 

Smooth     1  .  76 
Low           0  .  42 

These  differences  are  very  small  considering  that  the  yields 
obtained  averaged  between  60  and  70  bu.  per  acre.  Although 
continuous  selection  isolated  strains  which  differed  consider- 
ably from  each  other,  the  yields  were  not  markedly  affected. 
The  progressive  change  in  shelling  percentage  of  the  progeny  was 
most  striking  and  illustrates  how  corn  may  be  modified  by  selec- 
tion (see  Table  XLV). 


198 


BREEDING  CROP  PLANTS 


TABLE   XLV. — SHELLING    PERCENTAGE   AS   AFFECTED  BY  CONTINUOUS 

SELECTION 


Shelling  percentage  in  crop  harvested 

Year 

High 

Low 

1910 

84.73 

83.67 

1911 

87.30 

84.66 

1912 

85.34 

77.86 

1913 

87.09 

76.93 

The  results  presented  in  Table  XLIV  do  not  justify  the  belief 
that  selection  for  ear  type  is  a  means  of  improving  yield.  Other 
experiments  (Olson  et  al,  1918)  have  given  results  of  a  similar 
nature. 

Ear -to-Row  Breeding. — Corn  is  very  largely  cross-pollinated, 
therefore  selection  under  normal  conditions  considers  only  the 
mother  plant.  The  ear-to-row  method  has  been  considered  as 
the  quickest  means  of  isolating  an  improved  variety.  It  was 
first  introduced  by  Hopkins  (1899)  at  the  Illinois  Experiment 
Station.  As  East  (1908)  pointed  out,  the  method  has  some 
difficulties  which  have  been  partly  obviated  by  improvements  in 
technic.  The  improvements  consisted  of  replication;  i.e.,  dupli- 
cation of  rows  from  the  same  ear  in  different  parts  of  the  field;  and 
of  an  attempt  to  overcome  the  harmful  effects  of  too  close  in- 
breeding. The  method  outlined  by  Williams  (1905,  1907)  was  to 
plant  one-half  the  seed  of  each  ear  that  was  used  for  the  ear-to- 
row  test.  The  remnants  of  those  ears  which  excelled  by  the 
progeny  test  were  planted  and  the  progeny  intercrossed.  Another 
feature  of  Williams'  plan  was  to  influence  several  breeders  to  work 
with  the  same,  variety.  New  blood  was  then  introduced  into  the 
ear-to-row  plot  of  each  breeder  every  fourth  or  fifth  year  from  a 
grower  who  was  using  the  same  breeding  method.  The  difficulties 
of  the  method  are  that  a  yearly  plot  is  needed  for  the  ear-to-row 
test,  an  isolated  plot  for  the  crossing  of  the  remnants,  a  multi- 
plication or  seed  plot,  and  the  general  field.  Montgomery  (1909) 
suggested  a  plan  which  obviates  some  of  these  difficulties.  This 
plan  is  to  grow  an  ear-to-row  plot  only  once  in  several  years,  and 
in  the  intervening  years  use  a  bulk  seed  plot  planted  by  the  hill 
method,  selecting  only  from  the  vigorous  stalks  in  perfect  stand 
hills  (see  Chapter  XIX).  A  review  of  the  literature  on  ear-to- 


MAIZE  BREEDING 


199 


row  breeding  seems  unnecessary.  It  seems  sufficient  here  to  point 
out  that  there  are  no  experiments  which  show  conclusively  that 
continued  ear-to-row  breeding  may  be  expected  to  give  a  signifi- 
cantly higher  yield  than  seed  produced  by  the  seed-plot  method. 
Ear-to-row  breeding  with  a  variety  that  has  not  been  system- 
atically selected  is  doubtless  the  most  rapid  means  available  to 
the  corn  farmer  for  the  isolation  of  better  yielding  hereditary 
combinations.  As  an  illustration  of  the  sort  of  results  usually 
obtained,  the  results  of  a  five  years'  study  as  carried  on  at 
Nebraska  (Kiesselbach,  1916)  are  given  in  Table  XLVL 
TABLE  XL VI.  —  EFFECT  OF  EAR-TO-ROW  BREEDING  ON  THE  YIELD  OF 
HOGUE'S  YELLOW  DENT,  AT  THE  NEBRASKA  STATION,  1911-1915 


Yield  in  bushels  per  acre 

1911 

1912 

1913 

1914        1915 

Average 

Original  Hogue's  Yellow  Dent,.  . 

42.6 

51.6 

9.8 

62.8 

79.5 

49.3 

Continuous  ear-to  row  selection. 

44.0 

52.9 

7.7 

65.3 

76.8 

49.3 

Increase  from  single  ear-to  row 

strain. 

38  2     45  fi 

7  3 

tt  0 

75  3 

44  3 

Increase    from    composite   four 

ear-to-row  strains 

42.5 

54.6 

12.1 

63.5 

80.0 

50.5 

These  studies  with  Hogue's  Yellow  Dent  were  started  in 
1902.  This  variety  was  selected  because  of  its  yielding  ability  as 
shown  by  varietal  test.  Apparently  no  method  of  selection  has 
given  very  strikingly  beneficial  results. 

Home-Grown  Seed. — The  value  of  using  home-grown  seed  of  a 
variety  which  has  shown  its  yielding  ability  by  competitive  test  is 
well  known  to  most  corn  growers.  Nebraska  results  may  again 
be  used  for  illustrative  purposes. 

TABLE  XLVIL — EFFECT  OF  ACCLIMATIZATION  ON  CORN 


Character  of  seed 


Yield 

in  bushels 

per  acre 


Show  corn  from  Illinois,  Indiana,  and  Ohio  (5  varieties) 

Seed  from  growers  in  state  (5  varieties) 

Local  varieties  near  experiment  station  (7  varieties) 


39.8 
45.6 

48.8 


The  data  presented  in  Table  XLVII  show  that  home-grown 
seed  usually  yields  better  than  seed  brought  from  a  distance.     A 


200 


BREEDING  CROP  PLANTS 


system  of  broad  breeding,  the  use  of  a  high-yielding,  adapted 
variety,  and  the  storage  of  the  seed  so  that  it  will  germinate 
vigorously  are  important  practices  which  should  be  a  part  of  each 
corn-breeder's  plan. 

Relation  between  Heterozygosis  and  Vigor. — In  an  earlier 
chapter  the  effects  of  self-fertilization  in  corn  were  discussed 


FIG.  45. — Minn.  No.  13  self-fertilized  high  protein  strain  No.  1.  This  strain  has 
dark  green  leaves,  medium  sized  ears  and  the  tassels  are  somewhat  scantily  pro- 
vided with  pollen. 

and  the  hypothesis  outlined  that  vigor  in  FI  crosses  was  due  to 
the  partial  dominance  of  linked  growth  factors.  This  question  is 
of  considerable  importance  from  the  standpoint  of  the  corn 
breeder.  The  subject  will  be  discussed  under  the  following 
headings: 


MAIZE  BREEDING 


201 


1.  Immediate  effect  of  crossing  on  size  of  seed. 

2.  FI  varietal  crosses. 

3.  Isolation  of  homozygous  strains. 

Immediate  Effect  of  Crossing  on  Size  of  Seed. — The  question 
of  immediate  effect  of  crossing  on  size  of  seed  has  received  con- 
siderable attention,  and  Carrier  (1919)  has  recently  considered 
this  a  main  cause  for  the  conflicting  results  of  corn  experiments. 
He  demonstrated  the  fact  that  mixtures  of  seed  of  different  strains 
gave  higher  yields  than  seed  of  a  single  strain  and  explained  the 
results  on  the  basis  of  increased  yield  due  to  the  increased  weight 
of  the  endosperm  of  varietal  crosses  as  compared  with  normally 
pollinated  seeds  within  a  variety. 

Other  investigations  have  partially  supported  Carrier's  con- 
tentions. Studies  of  the  effect  of  pollen  of  a  different  strain  or 
variety  on  endosperm  development  are  given  in  Table  XLVIII. 

TABLE  XLVIII. — IMMEDIATE    EFFECT    OF    POLLINATION    ON    ENDOSPERM 

WEIGHT 


Num- 
ber of 
tests 

Number  in 
which  weight  of 
crossed  seed 
exceeds  that  of 
normal  seed 

Number  in 
which  weight  of 
normal  seed 
exceeds  that  of 
crossed  seed 

Average  per- 
centage of  in- 
crease due  to 
immediate  ef- 
fect of  foreign 
pollination 

Authority 

Method 

5 

5 

0 

8.8 

Collins    and 

Mixture  of  pol- 

Kempton,  1913     len     of     same 

and     different 

variety. 

31 

23 

8 

2.8 

Wolf,  1915 

Mixture  of  pol- 

len    of     same 

and     different 

variety. 

2 

2 

0 

19.2 

Jones,  1918 

Selfed      strains 

and    crosses 

between  them. 

These  results  show  that  there  was  an  immediate  effect  of  pollen 
on  the  weight  of  the  endosperm  of  crossed  seed  compared  with 
that  produced  by  intra-varietal  pollination.  In  varietal  tests, 
however,  as  conducted  by  the  plot  method,  the  degree  of  crossing 
between  different  varieties  would  not  usually  be  over  50  per  cent. 
Averaging  the  results  of  Wolf  and  of  Collins  and  Kempton  gives 
about  5  per  cent,  increase  due  to  crossing.  Reducing  this  by 
half  gives  an  error  in  varietal  tests  of  not  more  than  2.5  per  cent, 
as  a  result  of  increased  endosperm  development  due  to  the  im- 
mediate effect  of  foreign  pollen.  As  the  studies  of  Collins  and 


202  BREEDING  CROP  PLANTS 

Kempton  were  made  with  widely  different  varieties,  the  results 
are  probably  somewhat  more  striking  than  if  more  closely  re- 
lated forms  had  been  used. 

FI  Varietal  Crosses. — The  utilization  of  hybrids  as  a  means 
of  obtaining  more  vigorous  types  was  urged  by  Beal  (1876-1882). 
Since  then  there  has  been  frequent  mention  of  the  vigor  of 
FI  crosses,  and  Morrow  and  Gardner  (1893,  1894)  outlined  a 
plan  for  the  production  of  crossed  corn  seed.  Renewed  interest 
in  this  subject  was  aroused  as  a  result  of  the  publications  of  East 


FIG.  46. — Minn.  No.  13  high  protein  strain  No.  4.  Short,  erect  strain  with 
light  green  leaves.  Produces  good  ears.  Tassels  are  plentifully  supplied  with 
pollen. 

(19086)  and  Shull  (1908,  1909)  on  the  effects  of  inbreeding  and 
cross-breeding,  and  of  Collins  (1909,  1910)  on  the  value  of  first 
generation  hybrids  in  corn.  Many  experiments  in  which  first 
generation  crosses  have  been  compared  with  their  parents  have 
been  made.  In  Table  XL  IX  only  those  varietal  crosses  are  used 
in  which  the  FI  has  been  compared  with  both  parents. 

A  careful  study  of  this  table  shows  that  first  generation  crosses, 
on  the  average,  yield  more  than  the  average  of  their  parents. 
In  many  cases  the  cross  exceeds  the  higher  yielding  parent.  No 


MAIZE  BREEDING 


203 


•g 

"ft 

II 
i! 


• 

"ft  (« 

^s 


i  P 


o>    QJ    D.   ft 
T3 


r2  «       d        rf 

S    tits? 

„    'a'ft'ft 

'"So      ,2  ,2   ^ 

C          95  M 

w  «  .2  .2  2 

.2  02  02  02 

w        ^        -/          /        -/ 


If 


ll 


1*1 


ap 

est 


ft  ft 

73   T) 


T3  T3 
«  ^ 
ft  ft 

-a  -a 
<J  <5 


II 

03    >» 

CJ      ^ 

II 


88 


varietie 
Adapted 
Average 


O    O    O    O    t^    rH 

00  CO  rH  TjH  t»  O 
<N  O  00  rH      rH 


O    <N    <N 

CO    CO    CO 


O  rH  CO      CO 


rH         •  CO  CO       rH  rH 


Tf       rH  CO 


00  CO  CO  O  <N  10     CO 
rH  CD  'O  t~  rH  CO     1C 


CM 


t-    CO    CM 

co  o  d       •<*< 
+  +   I        + 


rH     T}<    CO    IN    <N    05  CO 


<M    00 


M<  N(Nt-CO<NO  »0 


CO    CO    O    10 


00  rH  rH    t^ 


QJ       OJ 

c  c 


2     ;  2 


t^  t-j    t>    S    rH  ft 


c     sf     < 


-I     s  S  §  2  g  i     i 

£  iiini  i 


ii 


S  s  .2 

r?    W    W 


£  «  S  2  S  S 

If  if r; 


^22 


•8 -8  'S  ft  o 


o  Q 


204 


BREEDING  CROP  PLANTS 


general  rule  can  be  given  and  the  only  sure  means  of  determining 
the  value  of  a  cross  is  by  the  experimental  test.  Results  have 
shown  that  FI  crosses  between  good  yielding  varieties  which 
differ  from  each  other  in  several  characters  frequently  yield  con- 
siderably more  than  either  parent  and  more  than  pay  for  the 
trouble  of  producing  crossed  seed.  Thus  the  tests  made  in  Con- 


FIG.  47. — Fi  cross  of  Minn,  self-fertilized  strains  No.  1  X  No.  4. 

necticut  (Jones  et  al,  1919)  and  those  carried  out  in  Minnesota 
(Hayes  and  Olson,  1919)  showed  thatFi  crosses  between  selected 
eight-rowed  flints  and  dents  very  frequently  exceeded  either 
parent  in  yielding  ability.  For  each  growth  character  in  which 
the  parent  varieties  differ  there  is  usually  an  intermediate  condi- 
tion in  FI.  There  is  a  tendency  for  a  partial  dominance  and  the 
first  generation  often  exceeds  the  average  of  the  parents  in  most 


MAIZE  BREEDING  205 

of  its  characters.  Fi  crosses  are  of  value  from  the  standpoint  of 
earliness.  Thus  a  cross,  studied  at  Minnesota  in  1919,  between 
Squaw  flint  and  Minnesota  No.  13,  approached  the  dent  parent 
in  height  of  plant  and  the  flint  parent  in  earliness  and  exceeded 
both  in  yield.  Such  a  cross  would  be  of  much  value  as  a  silage 
or  husking  variety  under  northern  conditions. 

The  production  of  crossed  seed  is  not  very  difficult.  The  va- 
rieties to  be  crossed  may  be  planted  in  alternate  rows  and  the 
tassels  removed  from  one  variety  before  any  of  the  pollen  has 
matured.  Seed  produced  by  the  detasseled  variety  is  known 
as  first  generation  crossed  seed.  If  the  varieties  to  be  crossed 
differ  in  maturity  they  should  be  planted  at  different  times  so 
that  both  bloom  at  about  the  same  date. 

Isolation  of  Homozygous  Strains.— Shull  (1908,  1909)  first 
suggested  the  utilization  of  crosses  between  self-fertilized  strains 
as  a  means  of  increasing  yield  in  corn.  Such  crosses  often  give 
very  high  yields.  The  chief  objections  to  this  method  are  that 
self-fertilized  strains  are  usually  of  very  low  yielding  capabilities 
and  that  the  seeds  from  self ed  lines  are  usually  much  smaller  than 
from  normally  pollinated  corn.  Even  though  crosses  between 
self -fertilized  lines  yielded  very  vigorously,  the  method  has  not 
seemed  commercially  desirable.  Low  yields  of  seed  per  acre 
would  increase  the  cost  of  seed.  Under  unfavorable  conditions 
the  food  supply  of  the  seed  might  not  give  the  young  F±  plant  a 
vigorous  start.  Jones  (1918)  has  made  a  suggestion  which 
removes  some  of  these  objections.  After  isolating  selfed  strains, 
tests  are  made  to  determine  which  four  biotypes  are  most  desir- 
able as  parents.  Suppose  these  are  numbered  1,  2,  3,  and  4 
respectively.  Numbers  1  and  2  are  crossed,  also  3  and  4,  by 
detasseling  all  of  one  biotype  in  each  group.  Seed  from  the 
plants  of  each  detasseled  biotype  is  then  planted  in  alternate 
rows  in  an  isolated  plot  and  all  of  one  combination,  as  3  X  4, 
detasseled.  Seed  from  these  detasseled  rows  is  used  for  com- 
mercial planting. 

This  method  seems  worthy  of  more  extensive  trial.  Such  a 
cross  was  compared  at  the  Connecticut  station  with  the  best 
dent  variety  obtained  from  a  varietal  survey  followed  by  a  vari- 
ety test.  The  highest  yielding  dent  variety  gave  a  yield  of  92  bu. 
while  the  cross  under  similar  conditions  yielded  112  bu. 

Every  investigator  who  has  produced  self-fertilized  strains  of 
corn  has  been  impressed  by  the  large  number  of  undesirable 


206  BREEDING  CROP  PLANTS 

abnormalities  which  are  isolated.  These  abnormalities  through 
ordinary  seed  selection  are  not  eliminated  from  the  commercial 
variety.  Self -fertilized  strains,  however,  stand  or  fall  upon  their 


FIG.  48. — Average  yields  of  4   self-fertilized   corn   strains   above;  F\  crosses  in 
the  center;  the  double  cross  below.     (After  Jones.) 

own  merits.  Through  self-fertilization  the  unfavorable  strains 
may  be  eliminated.  Crossing  of  the  more  desirable  strains 
followed  by  selection  seems  a  logical  method  for  synthetically 
producing  improved  maize  varieties. 


CHAPTER  XV 
GRASSES,  CLOVER,  AND  ALFALFA 

The  importance  of  hay  crops  in  the  world's  agriculture  makes 
desirable  their  consideration  from  the  standpoint  of  improve- 
ment by  breeding.  Grasses,  clover,  and  alfalfa  differ  strikingly 
in  (see  Chapter  III)  amount  of  seed  set  when  artificially  self- 
pollinated.  Red  clover  (Trifolium  pralense)  is  practically  self- 
sterile;  white  clover  (Trifolium  repens)  sets -few  seeds  when 
protected  from  insect  pollination;  timothy  (Phleum  pratense) 
under  a  bag  produces  few  seeds;  and  brome  grass  (Bromus 
arvensis)  under  the  same  conditions  sets  seed  abundantly. 
Although  common  alfalfa  (Medicago  saliva)  and  yellow  alfalfa 
(M.  falcala)  cross  freely,  seed  of  either  may  be  produced  by 
selfing.  Enough  examples  have  been  cited  to  show  that  there 
are  not  only  differences  in  the  modes  of  pollination  in  the 
three  mentioned  classes  of  hay  crops  but  also  differences  within 
each  class.  Carefully  controlled  experiments  with  grasses  to 
determine  the  percentage  of  naturally  crossed  and  naturally  self- 
fertilized  seed  are  very  limited.  To  what  extent  decrease  in 
vigor  will  result  from  artificial  self-pollination  is  also  an  un- 
answered question.  When  self-sterility  is  not  a  limiting  factor, 
the  methods  of  breeding  all  these  crops  are  essentially  alike. 
The  ease  with  which  some  of  them  may  be  clonally  reproduced 
has  led  to  slight  modifications  in  breeding  technic.  In  the 
following  brief  discussion,  the  aim  has  been  to  choose  a  few 
examples  rather  than  to  enter  into  an  exhaustive  treatment  of 
the  entire  field. 

GRASSES 

Timothy  ranks  far  ahead  of  the  other  grasses  in  importance. 
Some  of  the  other  hay  grasses  which  may  be  mentioned  are 
orchard  grass  (Dactylis  glomerala),  tall  oat-grass  (Arrhenatherum 
elalius),  and  brome  grass  (Bromus  inermis).  These  three  grasses 
are  adapted  to  certain  conditions  better  than  is  timothy.  Some 
important  pasture  grasses  are  Kentucky  bluegrass  (Poa  pralen- 

207 


208 


BREEDING  CROP  PLANTS 


sis),    Canada  bluegrass  (Poa  compressa),  arid  redtop  (Agrostis 
palustris) . 

The  variability  (see  Fig.  49)  of  each  of  the  different  species  of 
grasses  presents  a  wealth  of  material  for  breeding  purposes. 


FIG.  49. — Individual  timothy  plants  grown  under  like  conditions.  The  upper 
plants  are  undesirable,  one  having  weak  stems  and  the  other  lacking  vigor. 
The  lower  plants  are  more  desirable.  They  differ  in  density  of  plant  and  number 
of  culms.  (Courtesy  of  Myers.) 

Moreover,  the  fact  that  many  of  them  may  be  conveniently  prop- 
agated as  clones  facilitates  a  study  of  the  value  of  individual 


FIG.  50. — Flowers  of  timothy. 

1.  Spike. 

2.  Floret — a,    anther;    b,    filament;   c,    branched   stigma;   d,    style;   e,   ovary; 
/,  outer  glume. 

3.  Outer  glume. 

4.  a,  feathery  stigma;  6,  style;  c,  ovary. 

5.  Spikelet  showing  a,  palea;  b,  floral  glume.     (After  Seal  after  Trinius  and 
Scribner.) 

Size:  1,  $in;  2,  80n;  3,  4,  5,  greatly  enlarged.  '  "• 


GRASSES,  CLOVER,  AND  ALFALFA 


209 


a 


FIG.  50. 


210  BREEDING  CROP  PLANTS 

plant  selections.  The  hereditary  constancy  of  forms  so  isolated 
may  be  tested  by  selfing  or  by  adopting  methods  which  insure 
close  breeding. 

Breeding  Timothy. — The  United  States  Department  of  Agri- 
culture has  carried  on  extensive  experiments  in  timothy  breeding 
at  New  London  and  North  Ridgeville,  Ohio,  but  unfortunately 
the  work  has  not  been  published.  As  a  result  of  breeding,  two 
improved  varieties  have  been  widely  distributed  through  the 
Ohio  Experiment  Station.  The  Cornell  and  Svalof  Experiment 
Stations  have  done  considerable  timothy  breeding. 

Webber  et  al  (1912)  published  a  detailed  report  of  the  experi- 
ments as  carried  on  at  Cornell.  Samples  of  timothy  seed  were 
procured  from  various  sources  in  the  United  States,  Canada  and 
other  countries.  This  seed  produced  an  abundance  of  different 
forms  from  which  selections  were  made.  Individual  plants  were 
selected  on  the  basis  of  the  following  characters; 

1.  High-yielding  ability. 

2.  Height. 

3.  Broad  and  thick  plants,  which  stool  abundantly. 

4.  Many  and  dense  culms. 

5.  Erect,  non-lodging  plants. 

6.  Many  large  leaves. 

7.  Leaves  extending  well  toward  the  top  of  the  plant. 

8.  Leaves  remaining  green  until  plant  is  nearly  ready  to  harvest. 

9.  Rust  resistance. 

10.'  Spikes  of  medium  size,  setting  seed  freely. 

The  ultimate  aim  was  to  produce  a  high-yielding  variety. 
A  selected  plant  was  dug  up  and  vegetatively  propagated  by 
separating  bulblets  from  it.  The  bulblets  were  set  out  in  rows 
(16  to  24  per  row)  and  allowed  plenty  of  space  for  individual 
development.  Self-fertilized  seed  from  these  various  clones 
was  planted  in  sterilized  soil  and  the  seedlings  were  transplanted 
in  rows  as  above.  By  a  comparison  of  these  rows  and  the  re- 
spective clones  from  which  they  came  it  was  found  whether  they 
were  breeding  true  for  the  characters  desired.  When  sufficient 
seed  was  available,  plots  were  sown  broadcast  and  yields  obtained. 
As  soon  as  a  form  appeared  valuable  and  bred  comparatively 
true,  it  was  isolated  and  increased. 

According  to  Webber  self-fertilized  seed  may  be  produced  by 
placing  several  spikes  of  the  same  plant,  just  before  blooming, 
under  a  paper  bag.  At  University  Farm  Minn.,  only  a  few  seeds 


GRASSES,  CLOVER,  AND  ALFALFA 


211 


were  obtained  by  this  method.  Another  method  is  sometimes 
used  in  clonally  propagated  rows.  Each  row  is  surrounded  by 
a  fence  about  10  ft.  high  made  of  finely  woven  cloth.  This 
method  does  not  prevent  some  cross-pollination  but  it  does  bring 
about  a  high  degree  of  inbreeding.  A  tall  growing  crop,  such  as 
rye,  surrounding  isolated  plots  prevents  pollination  with  un- 
desirable strains  or  varieties. 

As  would  be  expected  in  dealing  with  a  heterozygous  crop, 
the  self-fertilized  seed  of  the  various  isolated  clones  produced 
plants  which  showed  considerable  difference  in  their  inheritance. 
Some  of  the  clones  bred  fairly  true  when  reproduced  by  selfed 
seed,  others  did  not.  Table  L,  taken  from  Webber  et  al  (1912), 
illustrates  the  transmission  of  yielding  ability  in  some  clones. 

TABLE  L. — TRANSMISSION  OF  YIELD  IN  TIMOTHY  BY  CLONAL    AND  SEED 

PROPAGATION 


Number  of 
original  plant 

Plat  No. 

Average  yield  per  plant 
of  mother  by  clonal 
propagation  (ounces) 

Plat  No. 

Average  yield  per  plant 
of  progeny  by  self- 
fertilized  seed  propaga- 
tion (ounces) 

LIGHT-YIELDING  PLANTS 


12.07 

1,797 

1.005 

3,216 

2.121 

9.03 

1,713 

1.830 

3,109 

3.364 

104.30 

1,794 

1.982 

3,213 

4.071 

191.19 

1,785 

2.283 

3,142 

3.143 

811.02 

1,728 

2.542 

3,166 

1.925 

128.19 

1,799 

2.462 

3,217 

0.966 

211.31 

1,792 

2.806 

3,211 

1.905 

212.36 

1,653 

2.811 

3,143 

4.140 

8.04            3,011 

2.941 

1,959 

3.714 

107.30            3,033 

3.158 

1,960 

1.182 

HEAVY-YIELDING  PLANT 


271.26 

1,660 

13.521 

3,152 

11.455 

887  .  10 

1,620 

13.783 

1,905 

7.600 

875.30 

,752 

13.811 

3,182 

7.915 

224.15 

,619 

14.133 

1,904 

9.000 

860.30 

,744 

14.517 

1,934 

7.636 

820.27 

,740 

15.587 

3,206 

10.844 

860.25 

,743 

15.970 

1,931 

9.428 

889  .  31 

3,189 

16  .  000 

3,190 

9.043 

245.28 

1,796 

16.308 

3,215 

9.457 

37.31 

1,630 

20.274 

3,122 

7.636 

212 


BREEDING  CROP  PLANTS 


The  practical  results  which  have  been  attained  by  this  method 
of  breeding  are  brought  out  in  Table  LI,  also  taken  from  Webber 
etal 

TABLE  LI. — SUMMARY,    SHOWING  YIELD  OF  FIELD-DRY  HAY 

Yield  in  pounds  per  acre 


1910 


1911 


Average  yield  of  17  new  varieties. 

Average  yield  of  7  checks 

Actual  average  increase 


7,451 

6,600 

851 


7,153 
4,091 
3,062 


FIG.  51. — View  of  vegetatively  propagated  row  plots  of  timothy.  Each  plot 
is  propagated  from  a  single,  original  plant.  Note  that  the  two  central  plots  are 
comparatively  late  in  maturity;  also  note  differences  between  these  two  strains, 
one  having  erect  culms  and  heads,  the  other  having  somewhat  spreading  culms 
and  long  loose  heads.  (Courtesy  of  Piper.) 

The  season  of  1911  was  particularly  unfavorable  for  the  growth 
of  timothy.  The  new  varieties  gave  a  greater  increase  that  year 
than  in  the  preceding  and  more  favorable  one.  Webber  et  al 
attribute  this  difference  partly  to  the  rust  resistance  of  the  new 
strains. 

The  method  of  breeding  timothy  at  Syalof  as  reported  by 
Witte  (1919)  is  not  essentially  different  from  that  practiced  at 


GRASSES,  CLOVER,  AND  ALFALFA 


213 


Cornell.  Individual  plant  selections  are  vegetatively  propa- 
gated in  plots  isolated  as  much  as  possible.  Seeds  produced  by 
the  better  clones  are  planted  in  varietal  plots  for  comparison. 
The  best  commercial  varieties  are  also  grown  for  comparison. 
When  a  new  variety  proves  superior  and  has  practical  uniformity, 
it  is  increased  and  distributed  on  a  large  scale.  A  comparison 
of  ordinary  timothy  and  two  improved  forms  distributed  by  the 
Svalof  Station  is  shown  in  Table  LII  (Witte,  1919). 

TABLE  LII. — YIELD  OF  DIFFERENT  VARIETIES  OF  TIMOTHY  IN  TRIALS  AT 
SVALOF,  1909-1918 


Variety 

Kilograms  of  green  fodder  per  hectare 

First  year's 
lay 

Second  year's 
lay 

Total 

Yield  per  cent, 
compared  to 
ordinary  Swedish 
timothy 

Svalof  s  Gloria  .... 
Svalof  's  Primus.  .  .  . 
Ordinary  Swedish.  . 

14.48 
13.46 
11.57 

11.03 
10.21 
9.59 

25.51 
23.67 
21.16 

120.6 
111.9 
100.0 

Timothy,  like  many  other  grasses,  is  susceptible  to  a  rust 
(Puccinia  graminis).  It  has  already  been  mentioned  that  in 
making  selections  at  the  Cornell  Station  some  attention  was 
given  to  resistance  to  this  fungus.  Eleven  of  the  better  Cornell 
selections  have  been  tested  for  rust  resistance  (Hayes  and 
Stakman,  1919).  The  relation  of  other  characters  to  resistance 
was  also  studied.  The  rust  classes  are;  1,  no  rust;  2,  slight  in- 
fection; 3,  moderate  infection:  and  4,  heavily  rusted.  Average 
erectness  is  taken  with  1  as  a  basis  of  an  erect  plant  and  10  a 
procumbent  one.  Table  LIII  presents  the  data. 

From  the  table  it  is  apparent  that  the  Cornell  selections 
possess  a  high  degree  of  resistance.  Relatively  few  plants  are 
found  in  rust  classes  3  and  4.  The  Minnesota  selections  show  the 
reverse  condition,  i.e.,  most  of  the  plants  are  found  in  classes  3 
and  4.  These  facts  show  that  a  variety  of  rust-resistant  timothy 
may  be  isolated. 

Timothy   breeding   may   be   briefly   summarized   as  follows: 

1.  Individual  plants  propagated  vegetatively  in  rows.     Bulb- 
lets  are  placed  far  enough  apart  in  the  row  to  give  ample  room 
for  individual  development. 

2.  The    clones   produced   in  1  are  closely  inbred  or  seed  is 
saved  from  vegetatively  multiplied  plants  in  isolated  plots.     By 


214 


BREEDING  CROP  PLANTS 


TABLE  LIII. — RUST  RESISTANCE  IN  TIMOTHY  IN  RELATION  TO  OTHER 
CHARACTERS  AS  SHOWN  BY  VARIOUS  DATA 


Variety 

Rust  classes 

a 

iB 
in 

Erectness 
mean 

•3* 

S  ;*"§ 
%"•* 

Average 
height,  cm. 

Average 
numbers  of 
stools 

1 

2 

3 

4 

Cornell  1,611  

80 

11 

l 

0 

1.1 

1.0 

2.3 

11.9 

88 

122 

Cornell  1,620  

77 

26 

3 

2 

1.3 

1.0 

2.2 

18.3 

88 

138 

Cornell  1,630  

79 

15 

9 

2 

.3 

0.9 

3.1 

12.3 

85 

141 

Cornell  1,635  

61 

14 

9 

3 

.5 

0.8 

2.9 

11.4 

85 

109 

Cornell  1,671  

56 

13 

13 

5 

.6 

0.8 

2.9 

11.6 

89 

123 

Cornell  1,676  

87 

10 

3 

6 

.3 

0.9 

2.9 

13.1 

87 

116 

Cornell  1  687 

86 

12 

9 

6 

.4 

0.9 

3.7 

11  .3 

88 

131 

Cornell  1,715       

90 

11 

6 

3 

.3 

0.8 

3.0 

10.3 

86 

98 

Cornell  1,743  

100 

3 

12 

7 

.4 

1.0 

5.6 

10.9 

84 

134 

Cornell  1,777  

36 

0 

4 

0 

.2 

0.9 

5.7 

11.5 

83 

142 

Cornell  3,230  

32 

5 

5 

0 

.4 

0.9 

3.1 

12.9 

86 

117 

U.  8.  Dept.  Sel.  1  ... 

2 

12 

70 

40 

3.2 

0.6 

2.6 

13.0 

91 

64 

U.  S.  Dept.  Sel.  2  ... 

4 

4 

19 

13 

3.0 

0.8 

3.3 

10.9 

87 

90 

U.  S.  Dept.  Sel.  3  ... 

0 

7 

15 

9 

3.1 

0.8 

2.8 

13.3 

92 

72 

L.  L.  May  Sel.  1  .... 

2 

3 

24 

15 

3.2 

0.6 

2.3 

12.4 

86 

70 

L.  L.  May  Sel.  2.... 

8 

5 

25 

6 

2.7 

0.6 

3.1 

10.1 

80 

92 

Griggs  Bros.  Sel.  1  .  . 

1 

1 

15 

5 

3.1 

0.7 

3.0 

12.8 

86 

91 

planting  the  seed  so  produced  clones  are  tested  for  transmission 
of  the  desired  characters  and  also  for  uniformity. 

3.  When  sufficient  seed  is  available,  plots  are  sown  broadcast 
and  tests  for  yield  are  obtained  under  ordinary  field  conditions. 

4.  A  selection  which  has  shown  performance  ability  is  in- 
creased in  isolated  plots  and  distributed  to  the  farmers. 

CLOVERS 

The  importance  of  clovers  as  forage  crops  and  their  role 
in  soil  improvement  make  them  of  great  economic  value.  Tri- 
folium  pratense,  or  ordinary  red  clover,  is  by  far  the  most  widely 
grown.  Alsike  clover  (T.  hybridum),  because  it  may  be  grown 
in  more  acid  soil  than  the  other  clovers,  is  favored  in  certain 
localities.  Some  of  the  other  clovers  are  white  (T.  repens), 
crimson  (T.  incarnatum) ,  and  the  sweet  clovers  (Melilotus  alba 
and  M.  officinalis).  All  of  these  species  are  biennial  or  peren- 
nial except  T.  incarnatum,  which  is  an  annual. 

Red  Clover. — It  has  been  demonstrated  several  times  that 
the  species  T.  pratense  will  set  practically  no  seed  when  protected 
from  the  visit  of  insects,  particularly  bumblebees.  However, 
this  is  not  the  only  factor  which  influences  fertilization.  West- 
gate  et  al  (1915)  found  that  moist  soil  and  atmospheric  condi- 


GRASSES,  CLOVER,  AND  ALFALFA  215 

tions  induced  the  formation  of  a  large  percentage  of  infertile 
ovules.  All  the  cells  remained  sporophytic,  no  reduction  taking 
place  with  the  formation  of  an  embryo  sac.  As  much  as  100 
per  cent,  ovule  infertility  was  found  in  the  first  clover  crop. 
The  rate  of  pollen  tube  growth  was  shown  to  be  much  slower  in 
self-  than  in  cross-pollinated  plants.  It  is  probable  that  pollen- 
tube  growth  is  too  slow  to  effect  fertilization  when  the  plant  is 
selfed.  The  pollen  of  red  clover  is  easily  burst  by  an  excess 
supply  of  moisture.  Martin  (1913)  demonstrated  that  good 
artificial  germination  of  pollen  could  be  obtained  on  membranes 
which  were  just  moist  enough  properly  to  regulate  the  supply  of 
water  to  the  pollen.  He  suggests  that  the  stigma  of  red  clover 
performs  the  same  function  as  the  membranes. 

The  above  facts  necessitate  a  method  of  breeding  which 
is  essentially  a  restricted  form  of  mass  selection.  Before  starting 
selection  it  is  desirable  to  make  comparisons  of  the  varieties  prod- 
uced by  other  breeders  and  of  commercial  seed  from  different 
sources  to  obtain  the  best  form  for  further  breeding  operations. 
A  seed  plot  may  then  be  used,  in  which  each  plant  is  spaced 
so  that  its  characters  may  be  determined.  Undesirable  plants 
should  be  removed  before  pollination.  By  repeating  this  process, 
forms  with  the  desired  characteristics  and  with  practical  uni- 
formity may  be  isolated. 

Selection  for  Disease -Resistant  Clover. — Clover  anthracnose 
(Colletotrichum  trifolii),  causes  serious  injury  to  red  clover  in 
certain  regions.  Bain  and  Essary  (1906)  issued  a  preliminary 
report  on  isolating  an  anthracnose  resistant  red  clover.  Healthy 
plants  in  a  badly  infested  field  were  located  late  in  the  season 
after  most  plants  had  been  killed  by  the  disease.  The  seeds  of 
the  chosen  plants  were  planted  separately  in  alternate  rows  with 
ordinary  commercial  seed.  Measures  were  taken  to  insure  the 
infection  of  every  seedling  with  anthracnose.  By  June  1  the 
commercial  plants  began  to  show  symptoms  of  the  disease  and 
by  the  middle  of  September  not  more  than  5  per  cent,  of  them 
were  living,  while  95  per  cent,  of  the  selections  were  healthy 
and  making  a  fair  average  growth.  Some  of  the  latter  showed 
small  lesions,  but  growth  was  not  seriously  injured. 

ALFALFA 

Alfalfa  is  one  of  the  oldest,  if  not  the  oldest,  plant  cultivated 
for  its  forage  only  (Piper,  1916).  Most  of  the  cultivated  forms 


216  BREEDING  CROP  PLANTS 

belong  to  the  species  Medicago  saliva.  The  only  closely  related 
species  of  economic  value  is  M.  falcata,  sometimes  called 
sickle  alfalfa  or  yellow-flowered  alfalfa.  The  two  species  cross 
readily,  as  Waldron  (1919)  has  shown  (for  pollination  studies  on 
alfalfa  see  Chapter  III.  Piper  et  al  (1914)  found  that  alfalfa  set 
more  seed  when  cross-pollinated  than  when  selfed,  although  the 
selfed  set  considerable  seed.  It  also  was  demonstrated  that 
automatic  tripping  with  consequent  self-pollination  may  occur 
under  certain  conditions. 

Grimm  Alfalfa  and  Winter  Hardiness. — Westgate  (1910)  and 
later  Brand  (1911)  suggest  that  the  origin  of  Grimm  alfalfa  is 
probably  the  result  of  natural  crossing  between  cultivated  alfalfa, 
M.  saliva,  and  wild  plants  of  the  yellow-flowered  sickle  lucern, 
M.  falcata,  found  especially  in  Germany,  Austria,  Roumania, 
and  certain  regions  of  Italy.  The  seed  from  which  the  Grimm 
variety  eventually  resulted  was  brought  to  Carver  County, 
Minnesota,  by  a  German  immigrant  farmer,  Wendelin  Grimm, 
in  1857.  Here  for  50  years  the  original  variety  was  subjected 
to  the  severe  Minnesota  winters  and  as  a  result  the  non-hardy 
types  were  eliminated.  At  the  present  time  Grimm  alfalfa  is 
probably  the  hardiest  variety  grown. 

Waldron  (1912)  reported  the  result  of  testing  for  winter  hardi- 
ness sixty-eight  different  strains  of  alfalfa  assembled  from  various 
parts  of  the  world.  The  trial  was  made  at  Dickinson,  N.  Dak. 
during  the  severe  winter  of  1908-09.  The  two  strains  of  Grimm 
alfalfa  included  in  the  experiment  proved  to  be  the  hardiest. 
On  an  average,  less  than  5  per  cent,  of  the  Grimm  plants  were 
killed  and  only  one  other  strain  showed  less  than  10  per  cent,  killed. 
Disregarding  twelve  strains  which  were  destroyed  completely,  the 
average  percentage  killed  for  the  other  strains,  considered  as  a 
unit,  was  77.5. 

To  bring  out  the  fact  that  differences  between  strains  in 
their  respective  reactions  to  cold  are  genetic,  Waldron  computed 

FIG.  52. — Structure  of  alfalfa  flowers. 

1.  Branch  showing  flowers  in  position. 

2.  Single   flower   showing — a,    standard;   6,    sexual   column   in   contact   with 
standard;  c,  keel;  dt  wings. 

3.  Seed  pod. 

4.  Flower    parts    in  position — a,  undeveloped  pod;  b,  ovary;  c,  filament;  d, 
anther.  . 

5.  Same  with  all  anthers  removed  except  one  to  show  stigma. 

6.  Anther. 

Size:  1,  about  %n;  2,  about  2n;  3,  about  }^n;  4,  5,  6,  greatly  enlarged. 


GRASSES,  CLOVER,  AND  ALFALFA  217 


6 


FIG.  52. 


218 


BREEDING  CROP  PLANTS 


correlation  coefficients.  Two  nurseries  had  been  planted  on 
succeeding  years  with  the  same  strains  taken  from  the  same 
original  lot  of  seed.  The  percentage  of  killing  of  the  various 
strains  in  one  nursery  during  the  winter  of  1908-09  was  correlated 
with  similar  data  collected  from  the  other  nursery  after  the  winter 
of  1910-11.  A  correlation  coefficient  of  +0.62  ±  0.06  was 
obtained. 

Some  of  the  surviving  plants  of  the  different  alfalfa  strains 
were  selfed  and  the  seeds  so  obtained  were  planted  separately  in 


FIG.  53. — Comparative  hardiness  of  Grimm  and  common  alfalfas.  The  two 
rows  in  the  center  are  from  Grimm  seed.  At  either  side  are  rows  grown  from 
southern  grown  common  seed.  1916  season.  (Photo  loaned  by  Arny.) 

a  third  nursery.  Percentage  of  winter  killing  of  these  strains 
was  taken  and  the. correlation  coefficient  between  the  percentage 
of  winter  killing  of  the  parental  stock  and  that  of  the  new  strains 
was  determined.  The  correlation  coefficient  obtained  was 
-f-0.46  ±  0.07.  The  mean  winter  killing  (expressed  in  per- 
centage) of  the  parental  stock  was  27.43  ±  1.75  as  compared 
with  6.43  ±  0.66  for  the  strains  coming  from  selfed  seed.  In 
other  words,  progress  has  been  made  toward  isolating  hardy 
biotypes. 


CHAPTER   XVI 
POTATO    IMPROVEMENT 

Potatoes  have  been  generally  introduced  into  cultivation 
since  the  discovery  of  America,  and  are  now  a  crop  of  major  im- 
portance in  many  countries.  The  large  number  of  varieties  is  an 
illustration  of  the  rapid  development  in  domestic  plants  of  varie- 
ties which  are  suited  to  special  soil  and  climatic  conditions.  As 
potatoes  are  reproduced  commercially  by  tubers,  they  furnish  an 
excellent  illustration  of  the  way  in  which  vegetative  reproduction 
modifies  breeding  methods. 

Origin  and  Species. — There  are  from  five  to  100  species  of 
tuber-bearing  potatoes  according  to  the  number  of  forms  which 
are  recognized  as  separate  species  (East,  19086:  Wight,  1916). 
Whether  the  cultivated  potato  arose  from  a  single  wild  species  or 
from  several  is  a  debatable  question.  The  preponderance  of 
opinion  is  that  there  is  only  a  single  wild  species,  Solanum  tubero-, 
sum  L.,  which  deserves  to  be  considered  as  the  stem  form  from 
which  all  cultivated  varieties  arose.  Wight  (1916),  after  care- 
fully examining  herbarium  material,  previous  records,  and  wild 
species,  makes  the  following  statements: 

"Every  reported  occurrence  of  wild  S.  tuberosum  that  I  have  been 
able  to  trace  to  a  specimen,  either  living  or  preserved  in  the  herbarium, 
has  proved  to  be  a  different  species.  I  have  not  found  in  any  of  the 
principal  European  collections  a  single  specimen  of  Solanum  tuberosum 
collected  in  an  undoubted  wild  state." 

Berthault  (1911)  cites  Heckel,  Planchon.  and  Labergerie  as 
examples  of  recent  workers  who  believe  that  other  wild  species 
gave  cultivated  S.  tuberosum  forms  by  mutation;  Planchon  be- 
living  that  the  original  form  was  S.  commersonii;  Heckel  that 
S.  maglia  through  mutation  produced  cultivated  potatoes;  while 
Labergerie  believed  both  of  these  species  gave  cultivated  forms 
through  mutation.  Berthault  attempted  to  answer  the  question 
by  growing  seeds  and  tubers  of  both  these  species  and  also  by 
growing  seed  of  several  cultivated  varieties.  Progeny  of  seed  or 
tubers  of  S.  maglia  and  S.  commersonii  gave  no  forms  which 

219 


220  BREEDING  CROP  PLANTS 

approached  in  calyx  or  corolla  characters  the  conditions  found  in 
S.  tuberosum  cultivated  varieties.  Progeny  of  seed  of  cultivated 
varieties  showed  Mendelian  segregation,  but  no  characters  were 
obtained  which  had  not  been  observed  in  ancient  cultivated 
varieties.  Wittmack  (1909),  after  a  careful  botanical  study  of 
species,  reached  the  conclusion  that  S.  tuberosum  was  the  stem 
species  from  which  all  cultivated  potatoes  arose. 

The  evidence  presented  by  De  Candolle  (1886)  seems  sufficient 
to  prove  that  the  potato  was  wild  in  Chile  and  in  a  form  which 
is  very  similar  to  that  of  our  cultivated  plants.  Heckel  (1912) 
reports  a  study  of  changes  under  cultivation  of  Solatium  tubero- 
sum forms  collected  in  the  wild  in  Bolivia  and  Peru  by  M.  Verne. 
The  wild  plants  were  0.25  meter  in  height,  bore  blue  flowers  and 
deep  green  foliage  and  tubers  about  the  size  of  a  hazel  nut  each 
produced  at  the  end  of  a  long  stolon.  These  tubers  were  planted 
at  Marseilles  in  a  garden  heavily  fertilized  with  manure.  Little 
change  was  observed  in  flower  and  fruit  characters  but  there  were 
pronounced  changes  in  the  subterranean  parts.  The  yellowish 
tubers,  each  borne  at  the  end  of  a  much  shortened  stolon,  con- 
tained a  much  greater  amount  of  starch  than  wild  tubers,  while 
the  characteristic  bitter  taste  of  the  wild  tubers  disappeared. 
Much  more  profound  changes  occurred  under  cultivation  with 
tubers  of  S.  maglia  (Heckel,  1909). 

There  seems  to  be  no  good  reason  for  speaking  of  all  these 
tuber  changes  as  mutations.  It  seems  more  in  line  with  modern 
genetic  usage  to  consider  them  as  the  normal  expressions  of  the 
inherited  factors  under  the  new  conditions  of  environment 
which  occur  under  cultivation. 

The  cultivated  potato  was  first  introduced  into  Spain  and 
Portugal  by  the  Spaniards  during  the  first  half  of  the  sixteenth 
century.1  Clusius  described  and  illustrated  the  potato  from 
plants  sent  him  in  1588  by  the  governor  of  Mons.  The  published 
description  was  made  in  Clusius'  "Rariorum  Plantarum  Histor- 
ia"  which  appeared  in  1601.  The  original  plant  obtained  by 
Clusius  bore  two  tubers  and  a  fruit  ball.  This  variety  bore  red- 
dish tubers  and  light  purple  flowers.  The  spread  from  this  in- 
troduction was  probably  next  into  Italy  and  from  there  early  in 
the  seventeenth  century  to  Austria,  then  to  Germany,  from  Ger- 
many to  Switzerland  and  then  to  France. 

Drake,  after  a  West  India  piratical  trip,  took  back  the  Roanoke 

1  EAST,  19086. 


POTATO  IMPROVEMENT  221 

colony  to  England,  including  Thomas  Herriott.  Probably  pota- 
toes were  part  of  the  stores  obtained  in  the  West  Indies  by  Drake 
and  these  Herriott  introduced  into  Ireland  about  1586.  This  was 
the  second  introduction  into  Europe,  the  Spaniards  deserving 
the  credit  for  the  first  introduction.  It  is  not  known  from  what 
source  the  English  colonists  of  Virginia  and  Carolina  first  obtained 
the  potato,  but  it  is  generally  believed  to  have  been  from  Spanish 
or  other  travelers.  Gerard  described  and  illustrated  the  potato 
in  his  "Herbal"  in  1597.  The  variety  he  described  possessed 
light  brown  to  yellowish  tubers  and  violet  to  almost  white 
flowers. 

Inheritance.1 — The  transmission  of  potato  characters  through 
the  seed  is  in  conformity  with  Mendelian  principles.  Vegetative 
propagation  allows  the  breeder  to  perpetuate  any  desirable  geno- 
type even  though  heterozygous,  which  is  the  usual  condition  in 
the  potato  plant.  While,  in  general,  self-fertilization  of  a  com- 
mercial variety  gives  rise  to  seedlings  which  vary  a  great  deal, 
it  is  comparatively  easy  to  obtain  homozygosity  for  some 
characters. 

Tuber  shape  and  size  are  important  characters  which  are  used 
as  one  means  of  varietal  classification.  Tuber  shape  has  been 
found  to  depend  essentially  on  the  presence  or  absence  of  a  single 
factor  for  length.  According  to  this  hypothesis  a  tuber  may  be 
homozygous  long,  homozygous  round,  or  heterozygous  long. 
Heterozygous  long  is  the  most  variable  of  the  three  conditions. 
In  one  experiment  two  varieties  with  round  tubers  when  selfed 
produced  nothing  but  round  tubers,  while  twelve  varieties  with 
oval  tubers,  when  selfed,  produced  long,  oval,  and  round  tubered 
progeny.  Nilsson  (1912-13)  found  one  variety  of  potato  that  did 
not  breed  true  for  round  tubers.  Long  tubers  were  dominant  to 
round  in  Fruwirth's  (1912)  experiments. 

Depth  of  eye  is  a  character  of  considerable  economic  impor- 
tance. In  general,  shallow  eyes  were  found  to  be  dominant  over 
deep  eyes. 

Several  factors,  in  addition  to  a  chromogen  body,  have  been 
recognized  in  tuber  coloration.  Red  potatoes  contain  two  genes, 
R,  a  reddening  factor,  and  D,  a  developer  of  pigment.  Purple 
and  black  tubers  have,  in  addition  to  R  and  D,  another  factor, 

1  The  following  discussion  is  based  on  inheritance  studies  made  by 
SALAMAN  (1909-11,  1910-11,  1911,  1912-13)  and  EAST  (19106)  except 
where  otherwise  noted. 


222  BREEDING  CROP  PLANTS 

P.  Segregating  ratios  were  in  accordance  with  the  above  fac- 
torial hypotheses.  Wilson  (1916)  obtained  only  white  tubers 
from  selfed  white-tubered  varieties.  Similar  results  have  been 
obtained  by  other  plant  breeders  which  show  that  white  is  a 
recessive  character.  A  certain  amount  of  coloring  in  the  young 
sprouts  or  shoots,  stems,  and  sometimes  in  the  leaf  petioles  was 
found  associated  with  the  presence  of  color  in  the  tubers.  With 
regard  to  flower  color,  three  white-flowered  varieties,  selfed, 
produced  only  white  flowers;  and  three  out  of  four  colored  varie- 
ties, when  selfed,  produced  both  colored  and  white  forms.  Color 
is,  therefore,  dominant  to  its  absence.  Inheritance  of  this 
character  may  be  explained  by  assuming  the  presence  of  a  chro- 
mogen  body  and  modifying  factors.  Heliotrope  flowers  are  due 
to  the  chromogen  body  plus  a  reddening  factor;  purple  flowers 
are  produced  by  the  addition  of  a  purpling  factor;  white  flowers 
may  be  due  to  the  absence  of  one  or  more  of  these  factors. 
Fruwirth  (1912)  found  red  tubers  dominant  over  white,  yellow 
flesh  over  white,  and  lilac-colored  flowers  over  white.  It  was 
also  found  that  different  gradations  of  color  were  inherited. 

Nilsson  (1912-13)  found  a  complicated  flower  color  inheritance. 
A  variety  with  violet-blue  flowers  gave,  on  selfing,  progeny  with 
red,  violet-blue,  near-red,  purple,  dark  and  light  blue,  and  white 
flowers.  A  variety  with  light  blue  flowers,  on  selfing,  yielded 
progeny  showing  simple  monohybrid  segregation  with  white 
recessive.  Evidence  that  several  factors  were  operating  in  the 
inheritance  of  tuber  flesh  color  was  also  obtained.  Some  of  the 
varieties  with  yellow  flesh  (tubers)  bred  true  when  selfed,  others 
segregated  as  dihybrids  with  white  recessive. 

The  inheritance  of  habit  of  growth  was  also  studied.  Plants 
may  be  upright,  bushy,  or  procumbent.  Bushy  plants  are 
heterozygous  for  habit  of  growth  and  many  of  them  exhibit  a  dis- 
tinct tendency  to  become  procumbent.  Homozygous  forms  of 
upright  and  sprawling  plants  may  be  isolated  easily.  Period  of 
maturity  is  used  as  a  means  of  varietal  classification.  It  is  prob- 
ably inherited  in  the  same  manner  as  with  other  crops. 

Sterility  of  the  anthers  has  been  found  to  be  a  dominant 
character.  At  first  Salaman  believed  that  its  inheritance  was  due 
to  a  single  differential  factor  but  later  evidence  indicated  a  more 
complex  manner  of  transmission.  Plants  producing  pale  helio- 
trope flowers  were  found  to  be  heterozygous  for  pollen  sterility. 

MacDougal   (1917)    crossed  the  wild  potato  of  Arizona,  S. 


POTATO  IMPROVEMENT  223 

fendleri,  which  grows  at  a  high  altitude  and  endures  extremes 
of  climate,  with  a  domestic  variety.  The  wild  form  produces 
small  tubers.  In  the  Fz  generation  forms  appeared  which  were 
identical  with  the  wild  parent  together  with  many  intermediate 
types. 

Most  of  the  observed  variations  in  cultivated  varieties  have 
occurred  in  the  tubers,  although  the  English  ash-leaf  varieties 
are  examples  of  a  variation  in  leaf  shape  (East,  19076). 

Production  of  New  Forms. — For  the  purpose  of  differentiating 
between  two  important  phases  of  potato  improvement,  Stuart 
(1915)  has  referred  to  "selection"  as  the  "isolation  and  asexual 
propagation  of  desirable  strains  or  types"  while  "breeding" 
is  used  only  for  sexual  reproduction.  With  certain  crops,  such 
as  the  potato,  this  terminology  is  distinctive.  Such  a  restricted 
usage  of  the  word  "selection"  seems  undesirable  from  the  plant- 
breeding  standpoint.  The  same  idea  can  be  obtained  by  the  use 
of  "clonal  selection"  to  refer  to  the  asexual  propagation  of  de- 
sirable strains  or  types. 

Systematic  plant  breeding  with  the  idea  of  combining  the 
desirable  characteristics  of  two  parental  varieties  can  be  carried 
out  only  after  the  breeder  has  familiarized  himself  with  the 
characters  of  particular  varieties  and  of  their  wild  relatives. 
Thus,  with  the  potato  as  with  other  crops  the  breeder  should 
first  determine  the  ideal  toward  which  he  will  work.  Parental 
varieties  should  than  be  selected  because  of  some  desirable 
characters.  By  recombination  of  the  favorable  characters  of 
both  parents,  improvement  may  be  obtained.  Gilbert  (1917)  has 
listed  certain  characters  of  the  potato  which  are  universally  de- 
sired. Some  of  these  are: 

1.  High  yield. 

2.  Good  quality. 

3.  Disease-resisting  capabilities. 

4.  Good  keeping  quality. 

5.  Good  color  of  flesh  and  skin. 

6.  Skin  of  desirable  texture. 

7.  Tubers  of  good  shape. 

8.  Shallow  eyes  relatively  few  in  number. 

9.  Upright,  vigorous  plants. 

10.  No  tendency  to  make  second  growth. 

The  desirability  of  most  of  these  characters  is  self-evident. 
The  chief  difficulties  in  the  way  of  developing  a  standardized 
method  of  attack  arise  from: 


224 


BREEDING  CROP  PLANTS 


1.  The  heterozygous  condition  of  most  varieties. 

2.  The  difficulties  of  obtaining  crossed  seed. 

The  heterozygous  condition  need  not  be  further  emphasized. 
Conditions  are  much  the  same  as  in  the  fruit  crops. 

The  Difficulties  of  Obtaining  Crossed  Seed. — The  technic 
of  making  a  cross  is  very  simple.  According  to  East  (1908a), 
"The  flowers  close  slightly  about  dusk  and  open  in  the  morning 
between  five  and  six  o'clock.  The  pollen  appears  to  be  in  the  best 
condition  for  use  on  the  second  day  of  blooming."  Stuart  (1915) 
collects  flowers  to  be  used  as  the  male  parent  in  small  sacks. 
After  the  pistil  is  removed  from  these  flowers  the  anthers  are 
tapped  sharply  with  a  pair  of  forceps,  the  pollen  is  collected 


FIG.  54. — Emasculated  and  unemasculated    potato    blossoms.     (After   Stuart.) 

on  the  thumb  nail  and  then  applied  to  the  pistil  of  the  emascu- 
lated flower.  The  flowers  are  receptive  two  to  four  days  after 
emasculation.  East  ( 1908a)  stated  the  belief  that  the  potato  is 
usually  self-fertilized.  He  also  observed  the  fact  that  insects  were 
seldom  seen  to  visit  the  flower.  Salaman  (1910-11)  believes 
it  unnecessary  to  cover  the  flower  before  or  after  pollination. 
Stuart,  however,  used  1-lb.  bags  and  found  that  if  a  certain 
amount  of  foliage  was  included  in  the  bag  the  use  of  bags  did 
not  cause  a  lowering  of  the  number  of  seeds  set.  An  average  of 
between  one  and  two  hundred  seeds  was  obtained  from  each  suc- 
cessful cross  by  Stuart. 

The  chief  difficulty  is  that  many  varieties  do  not  bloom  very 


POTATO  IMPROVEMENT 


225 


freely,  although  the  general  belief  is  that  all  varieties  may  bloom 
under  certain  conditions  of  environment.  East  (1908a)  classified 
varieties  as  follows: 

"1.  Varieties  whose  buds  drop  off  without  opening. 

"2.  Varieties  in  which  a  few  flowers  open,  but  which  immediately 
faU. 

"3.  Varieties  whose  flowers  persist  several  days,  but  which  rarely 
produce  viable  pollen. 

"4.  Varieties  which  under  most  conditions  always  produce  viable 
pollen." 

In  487  out  of  721  varieties  under  observation  the  buds  fell  off 
before  the  flowers  opened.  Stuart,  however,  obtained  a  much 
higher  percentage  of  varieties  which  produced  flowers  in  which 
the  blossoms  opened  before  the  buds  fell.  These  results  are 
given  to  emphasize  the  fact  that  conditions  widely  influence 
seed  production. 

The  lack  of  fertile  or  healthy  pollen  in  many  varieties  prohibits 
their  use  as  parents.  The  relation  between  the  percentage 
of  healthy  pollen  and  fruit  production  was  determined  by  East 
(1908a)  for  a  considerable  number  of  crosses  (see  Table  LIV). 

TABLE  LIV. — RELATION  BETWEEN  PERCENTAGE  OF  VIABLE  POLLEN  AND 
FRUIT  PRODUCTION 


Viability 

Fruit  production 

None 

Difficult 

Medium 
difficult 

Medium 

Good 

0-  25 
26-  50 
51-  75 
76-100 

per 
per 
per 
per 

cent, 
cent, 
cent, 
cent. 

healthy  

20 
5 

5 
2 

I 

3 

3 

6 

2 
3 

healthy  

healthy  
healthy  

Somewhat  similar  results  were  obtained  showing  a  positive 
correlation  between  fruit  production  and  the  percentage  of  multi- 
nucleate  pollen  grains.  Such  grains  may  be  determined  under 
the  microscope  by  their  slight  protuberances.  Germination  tests 
in  seven  per  cent,  sugar  solution  showed  that  a  pollen  tube  grew 
from  each  protuberance  in  a  multinucleate  grain.  These  results 
obtained  by  East  have  been  corroborated  by  the  studies  of 
Stuart.  In  some  cases,  however,  seed  production  is  not  difficult 
to  obtain  as  the  data  from  Stuart  show  (Table  LV). 

15 


226 


BREEDING  CROP  PLANTS 


TABLE  LV. — RESULTS  OP  POTATO  CROSSES  MADE  ON  THE  POTOMAC  FLATS, 
WASHINGTON,   D.  C.  IN  1910 


Parentage  of  cross 

Date 
emascu- 
lated 

Date 
pollin- 
ated 

Number 
of 
flowers 
crossed 

Number 
of  seed 
balls 
formed 

Num- 
ber of 
seeds 

Percent- 
age of 
seeds 
germi- 
nated 

Irish  Cobbler  X  Irish  Seedling  . 

July  28 

July  30 

6 

6 

964 

80.0 

Irish  Cobbler  X  Irish  Seedling  . 

July  28 

July  30 

7 

5 

984 

83.5 

Eureka  X  Keeper  

July  28 

July  30 

7 

5 

1,154 

78.3 

Improvement  Through  Seedling  Production. — Probably  no 
statement  could  be  more  illuminating  than  Stuart's  discussion 
regarding  early  studies  of  potato  improvement  in  the  United 
States.  The  facts  here  related  are  taken  from  Stuart's  pub- 
lication. 

During  the  period  from  1840  to  1847  the  wide  occurrence 
of  potato  blight  f ocussed  the  attention  of  potato  growers  upon  the 
need  of  more  resistant  varieties.  Rev.  C.  E.  Goodrich,  of  Utica, 
N.  Y.,  believed  this  susceptibility  to  diseases  was  a  result  of 
long-continued  asexual  propagation.  Through  the  agency  of  the 
American  consul  at  Panama,  South  American  varieties  were 
introduced.  Goodrich  grew  seedlings  from  Rough  Purple 
Chili,  one  of  the  introduced  varieties,  and  obtained  a  new  variety 
which  he  named  Garnet  Chili.  This  new  variety  was  introduced 
into  the  trade  in  1857.  Between  1849  and  1856  Goodrich  raised  a 
total  of  8,400  seedlings.  These  experiments  had  considerable 
effect  on  the  work  of  other  breeders.  In  1861  Albert  Bresee,  of 
Hubbardton,  Vt.,  grew  a  naturally  fertilized  seed  ball  produced 
by  Garnet  Chili.  One  of  the  seedlings  produced  was  distributed 
under  the  name  of  Early  Rose.  The  most  careful  breeder  of  this 
period  was  C.  C.  Pringle,  of  Charlotte,  Vt.  He  selected  varieties 
for  crossing  because  of  desirable  characters.  A  variety  by  the 
name  of  Snowflake  was  one  of  the  best  known  of  his  productions. 
Pringle,  in  the  early  seventies,  contracted  to  produce  potato  seed 
for  $1,000  a  pound. 

Numerous  varieties,  probably  resulting  from  naturally  polli- 
nated seed,  were  introduced  by  E.  S.  Brownell,  of  Essex  Center, 
Vt.  Among  the  better  known  of  these  were  Brownell's  Best, 
Beauty,  Eureka,  and  Winner.  Among  other  well-known  varie- 
ties which  were  introduced  about  this  time  was  the  early  maturing 
variety,  Early  Ohio.  Alfred  Reese  produced  this  variety,  which 


PO  TA  TO  IMPRO  YEMEN  T 


227 


was  introduced  in  1875,  from  a  seedling  of  Early  Rose.  One  of 
the  first  plant  productions  of  note  of  that  celebrated  breeder, 
Luther  Burbank,  was  obtained  by  growing  seedlings  of  a  potato 
ball  which  he  found  on  an  Early  Rose  vine  in  his  mother's  garden 
at  Lancaster,  Mass.  Of  23  seedlings  grown,  one  was  of  much 
promise.  This  was  introduced  by  Gregory  in  1872  as  Burbank's 
Seedling.  From  naturally  fertilized  seed  of  Garnet  Chili, 


FIG.  55. — An  extra-promising   first-year  seedling.     Crop   of   1910.     24   tubers. 

(After  Stuart.) 

E.  L.  Coy  of  West  Hebron,  N.  Y.,  obtained  a  variety  that  was 
introduced  in  1878  as  Beauty  of  Hebron.  These  early  experi- 
ments which  produced  some  varieties  that  are  still  grown  illus- 
trate the  marked  effect  which  the  introduction  of  a  single  variety 
may  have  on  the  production  of  new  forms.  Some  of  the  varieties 
which  resulted  from  the  introduction  and  breeding  experiments 
of  Rev.  C.  E.  Goodrich  are  here  listed : 

TABLE  LVI. — PEDIGREES  OF  SOME  POTATO  VARIETIES 


Breeder 


Variety  used  for  seed 


Seedlings  named 


Rev.  C.  E.  Goodrich  .  .  . 
Albert  Bresee  

Rough  Purple  Chili 
Garnet  Chili 

Garnet  Chili 
Early  Rose 

Alfred  Reese  
Luther  Burbank  
E.  L.  Coy  

Early  Rose 
Early  Rose 
Garnet  Chili 

Early  Ohio 
Burbank 
Beauty  of  Hebron 

228 


BREEDING  CROP  PLANTS 


These  early  studies  illustrate  the  general  mode  of  production 
of  new  potato  varieties.  Certain  methods  are  of  value  in  giving 
the  seedlings  a  good  start.  In  the  latitude  of  Washington, 
Stuart  recommends  sowing  the  seed  in  the  greenhouse  early  in 
March  and  transplanting  the  seedlings  from  3-in.  pots  into  the 
field  in  May.  The  plants  are  placed  in  rows  3  ft.  apart  and 
spaced  at  a  distance  of  2  ft.  apart  in  the  row.  Results  indicate 
that  seedlings  producing  tubers  of  irregular  shape  or  those  with 
deep  red  or  purple  skin  may  well  be  discarded  after  the  first 
year's  trial.  After  another  year's  study  those  strains  with 
undesirable  characters  such  as  low  yielding  ability,  undesirable 
shape,  deep  eyes,  unusual  susceptibility  to  fungous  diseases  and 


FIG.    56. — An    unpromising    first-year    seedling.     Crop    of    1910.     Note 
large  number  of  small,  irregular  shaped  tubers.     (After  Stuart.) 


the 


straggling  or  weak  vine  growth,  should  be  discarded  and  the  few 
more  promising  types  given  a  wide  test  to  determine  their 
adaptability  and  value  under  different  conditions. 

Clonal  Selection. — The  subject  of  bud  mutations  in  potatoes 
is  a  somewhat  difficult  one,  for  there  are  numerous  reported  cases 
of  such  sudden  changes.  Many  of  the  experiments  were  not 
performed  with  sufficient  care  to  furnish  acceptable  evidence, 
although  numerous  apparently  authentic  cases  of  color  changes 
have  been  reported.  As  an  illustration  of  carefully  controlled 
experiments  those  carried  on  by  East  (1910a)  may  be  cited.  In 
these  studies  each  variety  worked  with  was  started  from  a  single 
hill.  During  the  course  of  the  study,  five  permanent  changes 


POTATO  IMPROVEMENT 


229 


from  pink  to  white  tubers,  two  permanent  changes  from  long  to 
round  tubers,  and  four  instances  of  changes  from  shallow  to  deep 
eyes  were  observed.  On  the  basis  of  the  modes  of  inheritance  of 
these  characters,  the  hypothesis  was  made  that  the  changes 
resulted  from  the  loss  of  dominant  factors.  Experiments  in 
selection  for  high  nitrogen  content  gave  negative  results.  The 
statement  was  made,  "it  is  true  that  all  of  the  asexual  variations 
have  been  losses  of  characters,  while  in  sexual  reproduction  the 
formation  of  new  characters  occurs."  This  certainly  substan- 
tiates the  belief  that  the  production  of  improved  varieties  of 
potatoes  through  bud  mutation  is  not  a  promising  method  of 
attack.  East  quotes  A.  W.  Button,  who  states: 

"I  have  no  hesitation  in  affirming  that  there  is  no  potato  in  commerce 
in  England,  and  I  might  say  in  Europe,  which  owes  its  origin  as  a  dis- 
tinct potato  to  bud  variation  in  any  form  whatever." 

If  this  statement  is  true,  it  seems  fair  to  conclude  that  there  has 
been  a  somewhat  loose  usage  of  the  term  "bud  mutation"  in  its 
application  to  raising  the  standard  of  a  variety  by  any  of  the 
well-known  methods  such  as  tuber  unit  or  hill  selection  (see 
Chapter  XVIII).  Accumulated  evidence  certainly  points  to  the 
belief  that  the  chief  value  of  such  work  rests  on  the  prob- 
able elimination  of  degenerate  strains.  Evidence  from  Canada 
presented  by  Macoun  (1918)  is  particularly  illuminating.  Four 
varieties,  Early  Rose,  State  of  Maine,  Empire  State,  and  Dela- 
ware, were  grown  in  Canada  at  the  Experimental  Farm  at  Ottawa 
from  1890  to  1909  inclusive.  The  better  tubers  were  selected 
from  each  year's  crop  and  used  to  plant  the  following  crop. 
Results  are  presented  in  Table  LVII. 

TABLE  LVII. — AVERAGE  YIELD  OF  POTATOES  OVER  THE  FIRST  FOUR  AND 

LAST  FOUR  YEARS  OF  A  16-YEAR  PERIOD  AND  SUBSEQUENT 

YEARLY  YIELDS  OVER  A  FOUR-YEAR  PERIOD 


Year 

Variety 

1890-1893, 
bu. 

1902-1905, 
bu. 

1906, 
bu. 

1907, 
bu. 

1908, 
bu. 

1909, 
bu. 

Early  Rose  

257 

317 

150 

128 

69 

18 

State  of  Maine..  .  . 
Empire  State  
Delaware  

325 
301 
296 

361 
338 
352 

132 
132 
103 

174 
117 
114 

97 
117 
156 

62 
62 
53 

Average  

295 

342 

129 

133 

110 

49 

230  BREEDING  CROP  PLANTS 

For  the  16-year  period  from  1890  to  1905,  inclusive,  the 
varieties  were  kept  in  a  high  state  of  productivity  "due,  no 
doubt,  to  careful  selection  and  good  cultivation  each  year. "  In 
1906,  however,  there  was  a  marked  falling  off  in  yield  due  to  the 
unfavorable  season.  In  the  early  part  of  the  season  there  was 
sufficient  rain  but  at  about  the  time  of  the  last  cultivation,  hot 
dry  weather  set  in  and  continued  throughout  the  season.  During 
July  there  was  also  a  severe  attack  by  aphis.  The  vines,  therefore, 
presented  a  stunted  appearance  and  dried  up  early  in  the  fall, 
the  yield  of  tubers  being  very  low.  In  1907  and  1908  the  seasons 
were  also  very  unfavorable.  The  best  tubers  were  again  planted 
in  1909  and  although  the  tubers  used  for  planting  presented  a 
very  favorable  appearance,  the  yields  were  very  low.  A  com- 
parison was  made  in  1909  of  tubers  grown  continuously  at  the 
Central  Experiment  Farm  and  newly  imported  tubers  grown  under 
more  favorable  conditions.  The  yielding  ability  of  the  imported 
tubers  exceeded  that  of  the  Central  Farm  tubers  by  as  high  as 
500  per  cent,  in  some  cases. 

The  plant  breeder  is  naturally  interested  in  the  subject  of 
whether  these  are  instances  of  bud  variations  due  to  unfavorable 
environment.  If  so,  they  should  be  permanent  changes.  If, 
on  the  other  hand,  they  are  non-heritable  variations,  this  does 
not  affect  the  practical  importance  of  tuber  selection  as  a  means 
of  obtaining  high  yields.  Macoun  (1918)  has  furnished  evidence 
which  helps  to  clarify  our  ideas  on  this  question.  From  time  to 
time  tubers  were  sent  from  Ottawa  to  the  branch  stations,  on  the 
prairies,  where  potatoes  usually  grow  very  vigorously.  In  1916 
the  following  question  was  asked: 

"You  will,  no  doubt,  remember  that  potatoes  sent  you  from  Ottawa 
are  usually  weak  growers  when  you  receive  them.  I  would  be  glad  if 
you  would  inform  me  for  how  many  seasons  that  weak  growth  con- 
tinues, or  do  they  make  a  strong  growth  the  next  year,  the  same  as 
the  ones  you  have  been  growing  for  several  years?" 

Answers  made  by  the  superintendents  of  these  prairie  farms 
showed  that  the  first  year's  crop  from  tubers  sent  from  the  Cen- 
tral Farm  was  very  small.  From  one  to  three  years  elapsed 
before  varieties  introduced  from  the  Central  Experimental 
Farm  yielded  as  well  on  the  prairies  as  those  varieties  which  had 
been  continually  grown  on  the  prairies. 

Much  of  the  so-called  "running  out"  or  degeneracy  in  pota- 


POTATO  IMPROVEMENT  231 

toes  has  been  traced  to  certain  plant  diseases  (Stewart,  1916; 
Orton,  1914)  which  have  been  variously  named  as  leaf  roll, 
mosaic,  and  curly  dwarf.  Quanjer  (1920)  has  presented  evidence 
to  show  that  these  three  diseases  may  be  stages  of  the  same 
disease,  which  is  transmissible  from  plant  to  plant.  Similar 
results  have  been  obtained  at  University  Farm.1  The  disease 
is  called  " mosaic  dwarf"  by  Krantz  and  Bisby  in  unpublished 
investigations.  That  rejuvenation  of  a  variety  is  possible 
through  its  introduction  and  growth  under  a  more  favorable 
environment  is  illustrated  by  studies  which  have  been  carried  on 


FIG.  57. — Progeny  of  single  tubers  as  grown  at  University  Farm,  1918. 
Some  tubers  give  vigorous  progeny,  others  produce  only  small,  weak,  degenerate 
plants.  (Courtesy  of  Krantz.) 

cooperatively  between  the  Division  of  Horticulture  of  the  Min- 
nesota Central  Station  and  the  sub-stations.  Yields  for  Min- 
nesota No.  2  at  University  Farm  for  1914,  1915  and  1916, 
respectively,  were  196,  169  and  22  bu.  This  shows  the  rapid 
reduction  in  yield  which  is  obtained  by  the  continued  use  of 
tubers  saved  at  University  Farm.  Tubers  of  Minnesota  No. 
2  saved  from  the  1916  University  Farm  plot  gave  a  yield  cf 
170  bu.  at  Duluth  in  1917.  Tubers  from  this  Duluth  plot  yielded 
300  bu.  at  Grand  Rapids  in  1918.  Whether  a  badly  diseased 

1  Data  on  running  out  and  on  field  experiments  in  Minnesota  were 
furnished  by  F.  A.  KRANTZ  of  the  Division  of  Horticulture,  Minnesota 
Experiment  Station. 


232 


BREEDING  CROP  PLANTS 


variety  can  be  rejuvenated  by  planting  under  a  favorable  environ- 
ment tubers  from  diseased  plants  is  as  yet  an  unanswered  question. 
The  rapid  deterioration  of  varieties  when  University  Farm 
tubers  are  used  for  their  propagation  is  believed  to  result 
from  these  transmissible  diseases  which  are  now  called  "  mosaic 
dwarf."  Degeneracy  can  apparently  be  prevented  by  covering 
that  part  of  the  field  in  which  tubers  are  to  be  saved  for  the  next 
year's  planting  by  a  cheese-cloth  cover.  The  following  data 
with  the  variety  Green  Mountain  seem  sufficient  authority  for 
this  statement: 

TABLE  LVIII. — DEGENERACY  PREVENTED  BY  USING  TUBERS  OF  VINES  WHICH 
WERE  COVERED  WITH  A  CHEESE  CLOTH  COVER 


Source  of  timber  used 

Year 
grown 

Yield 
per  acre, 
bu. 

Probable 
error  in 
per  cent. 

Grown  in  open  at  University  Farm,  1917.  .  .  . 
Grown  under  cheese-cloth  cover  at  University 
Farm,  1917 

1918 
1918 

172 
223 

4.6 
5  4 

Newly  introduced  stock  

1918 

205 

1  7 

Grown  in  open  at  University  Farm,  1917  and 
1918  

1919 

1.5 

Grown  under  cheese-cloth  cover  at  University 
Farm   1917  and  1918                    

1919 

285 

4  8 

Newly  introduced  stock 

1919 

272 

1  5 

All  seed  stock  was  obtained  from  the  same  grower  at  the  North 
Central  Experiment  Station,  Grand  Rapids,  Minn.  Whether  all 
degeneracy  is  due  to  such  transmissible  diseases  is  as  yet  un- 
answered. Possibly  unfavorable  cultural  conditions  may  also 
affect  the  development  of  the  tuber  so  that  the  yield  of  the  fol- 
lowing year's  crop  may  be  modified. 

Another  explanation  of  degeneracy  has  been  commonly 
mentioned.  This  is  the  hypothesis  that  continued  asexual 
propagation  causes  senility  or  degeneracy.  Perhaps  this  question 
may  be  answered  for  the  potato  by  the  consideration  of  a  fact 
reported  by  Heribert  Nilsson  (1913).  In  a  report  of  yields  of 
67  varieties,  as  tested  in  Sweden,  he  emphasized  the  fact  that  a 
variety  "Hvit  Jamtlandspotatis"  which  has  been  cultivated 
more  than  100  years  proved  to  be  the  highest  yielder.  This  is 
given  as  a  refutation  of  the  theory  of  senility. 

It  has  not  been  the  intention  in  this  discussion  to  lead  the 


POTATO  IMPROVEMENT 


233 


practical  breeder  to  discard  "clonal  selection"  as  one  means  of 
obtaining  high  yields,  for  it  is  a  recognized  fact  that  seed  plot 
methods  are  of  much  practical  importance.  The  results,  how- 
ever, are  probably  not  due  to  the  isolation  of  bud  mutations  but 
rather  to  the  use  of  tubers  which  have  developed  normally  and 
which  furnish  the  right  conditions  to  give  the  resultant  plants 
a  favorable  start.  May  not  the  conditions  be  much  the  same 
as  with  any  vegetatively  propagated  plant.  Bonnier,  for 
example,  found  that  about  three  years  are  required  before  a  low- 


FIG.  58. — Tubers  produced  under  such  a  cheesecloth  cover  have  given  good 
yields  during  the  seasons  1918  and  1919  while  tubers  from  uncovered  vines 
produced  very  inferior  yields.  University  Farm,  St.  Paul,  Minnesota.  (Cour- 
tesy of  Krantz.) 

land  dandelion  transported  to  alpine  conditions  fully  expresses 
the  characters  of  a  dandelion  plant  which  had  been  grown  under 
these  conditions  for  many  years.  On  returning  the  same  plant 
to  the  lowlands  about  the  same  number  of  years  elapsed  before 
the.  plant  had  again  fully  attained  the  lowland  habit.  This  is 
probably  not  a  germinal  change  but  the  normal  expression  of  the 
plant  under  a  particular  environment.  With  the  clonally 
propagated  potato  there  is  a  cumulative  response  to  unfavorable 
conditions.  Such  conditions  modify  the  plant's  development 
and  therefore  influence  the  development  of  the  following  year's 
crop.  There  seems  no  reason  for  believing  that  an  actual  ger- 
minal mutation  has  occurred. 


CHAPTER  XVII 

BREEDING  OF  VEGETABLES 

SELF-FERTILIZED   VEGETABLES 

The  long  periods  of  cultivation  and  the  various  environments 
to  which  many  of  our  vegetables  have  been  subjected,  have 
served  to  increase  the  number  of  varieties.  Most  of  the  vegetable 
varieties  have  been  produced  by  commercial  seed  firms  or  by 
seed  growers.  An  examination  of  any  seed  catalog  shows  numer- 
ous new  forms  which  are  being  constantly  introduced  into 
cultivation.  There  has  been  a  marked  tendency  among  seeds- 
men to  give  new  trade  names  to  old  standard  varieties.  This 
has  led  to  a  great  deal  of  confusion  in  nomenclature  and  much 
difficulty  has  been  experienced  in  varietal  identification.  There  is 
need  of  a  more  scientific  test  of  varieties  prior  to  introduction  and 
of  a  standardization  of  varieties.  Considerable  progress  has  been 
made  in  classification  of  some  vegetables.  More  information 
is  needed  regarding  the  mode  of  pollination  and  inheritance  of 
special  characters  before  methods  of  breeding  can  be  intelligently 
applied.  In  this  chapter  the  origin  of  both  cross-  and  self- 
fertilized  vegetables  is  briefly  summarized.  The  mode  of 
inheritance  of  special  characters  of  the  self-fertilized  vegetable 
species,  pea,  bean,  tomato,  and  pepper  are  given,  together  with 
a  brief  discussion  of  methods  of  breeding. 

Origin  of  Vegetables.1 — The  ancient  Greeks  and  Romans  were 
familiar  with  some  of  our  garden  vegetables;  on  the  other  hand, 
many  are  of  more  recent  origin  and  new  varieties  are  being 
constantly  introduced.  The  discovery  of  America  introduced 
to  civilization  such  important  vegetables  as  the  Irish  potato, 

1  For  a  complete  history  of  the  origin  of  vegetables  see  DE  CANDOLLE,  A., 
Origin  of  Cultivated  Plants,  Kegan  Paul,  Trench  &  Co.,  London,  Second 
Edition;  468  pages,  1886;  HENSLOW,  G.,  The  Origin  and  History  of  Our 
Garden  Vegetables  and  Their  Dietetic  Values,  in  Jour.  Roy.  Hort.  Soc.,  vols. 
36  and  37,  1910-11  and  1911-12;  STURTEVANT,  E.  L.,  History  of  Garden 
Vegetables,  in  Am.  Nat.,  vols.  23,  24,  and  25,  1889,  1890,  and  1891. 

234 


BREEDING  OF  VEGETABLES 


235 


sweet  corn,  tomato,  bean,  sweet  potato,  pumpkin,  squash,  and 
pepper.  Nearly  all  the  other  cultivated  vegetables  of  temperate 
climates  are  indigenous  to  Europe  or  Asia. 

Sweet  corn,  which  is  one  of  the  most  highly  prized  foods 
grown  in  America,  is  probably  of  recent  origin.  In  Bailey's 
(1900)  Cyclopedia  of  American  Agriculture,  Volume  2,  page 
402,  the  following  statement  occurs: 

"The  first  sweet  corn  cultivated  in  America  was  derived  from  the 
Susquehanna  Indians  in  1779  by  Captain  Richard  Begnall,  who  accom- 
panied General  Sullivan  on  his  expedition  to  subdue  the  Six  Nations." 

How  long  Zea  mays  saccharata  had  been  under  cultivation  is 
TABLE  L1X. — ORIGIN  AND  ANTIQUITY  OP  SOME  VEGETABLES 


Vegetable                      Botanical  name                                Probable  origin 

Years 
culti- 
vated 

Asparagus  

Asparagus  officinalis 

Europe,  west  temperate  Asia.             '    B 

Bean,  lima  

Phaseolus  lunatus 

Brazil.                                                            E 

Bean,  common  .... 

P.  vulgaris 

S.  Am.  found  in  Peruvian  tombs. 

Ed) 

Beet  Beta  vulgaris 

Canaries,      Mediterranean      basin, 

B 

Tops  as  food 

western  temperate  Asia. 

Roots  as  food 

A  result  of  cultivation.                              B 

Cabbage  Brassica  oleracea 

Europe                                                          A 

Carrot  Daucus  carota 

Europe,  west  temperate  Asia  (?). 

B 

Celery  j  Apium  graveolens 

Temperate   and   southern    Europe, 

B 

:  » 

northern  Africa,  western  Asia. 

Corn,  sweet  Zea  mays  var.  saccharata 

Cucumber  Cucumis  sativus 

India.                                                          A 

Lettuce  

Lactuca  sativa 

Southern  Europe,  northern  Africa,      B 

western  Asia. 

Muskmelon     

Cucumis  melo 

India,  Beluchistan,  Guinea 

C 

Onion  

Allium  cepa  ....    

Persia,     Afghanistan,    Beluchistan, 

A 

Palestine  (?) 

Parsnip   

Pastinaca  sativa  

Central  and  southern  Europe. 

C 

Pea,  garden  

Pisum  sativum 

From  the  south  of  the  Caucasus  to 

B 

Persia  (?),  northern  India  (?). 

Pepper   

Capsicum  annuum 

Brazil  (?) 

E 

Pumpkin  

Cucurbita  pepo 

Temperate  North  America. 

E 

Radish    

Raphanus  sativus 

Temperate  Asia. 

B 

Salsify   

Tragopogon  porrifolium 

Southeast  of  Europe,  Algeria. 

C  (?) 

Spinach  

Spinacia  oleracea 

Persia  (?).     , 

C 

Sweet  potato  

Convolvulus  batatas 

Tropical  America  (where?). 

D 

Tomato       

Lycopersicum  esculentum 

Peru. 

E 

Turnip   

Brassica  rapa 

Europe,  western  Siberia  (?) 

A 

Watermelon  

Citrullus  vulgaris 

Tropical  Africa. 

A 

A  =  Species  cultivated  more  than  4,000  years. 

B  =  Species  cultivated  more  than  2,000  years. 

C  =  Species  cultivated  less  than  2,000  years. 

American  species: 

D  =  Cultivation  ancient  in  America. 

E  =  Cultivation  before  discovery  of  America,  but  not  showing  signs  of  great  antiquity. 


236  BREEDING  CROP  PLANTS 

not  known,  but  there  is  considerable  evidence  to  substantiate 
the  belief  that  at  least  the  main  types  of  corn,  Zea  mays  indentata 
and  Zea  mays  indurata,  were  cultivated  a  long  time  before  the 
discovery  of  America. 

Table  LIX  taken  from  De  Candolle  (1886)  presents  a  summary 
of  the  origin  of  some  common  vegetables. 


PEAS 

Some  Classification  Characters. — Considerable  historical  in- 
terest attaches' to  the  pea  because  of  the  fact  that  in  studying 
the  inheritance  of  certain  characters  in  this  plant  Mendel  dis- 
covered his  now  famous  principles.  Garden  peas  (Pisum  sati- 
vum)  are  of  two  kinds,  shelling  and  edible-pod.  In  the  former, 
seeds  only  are  used  as  food,  while  in  the  latter  both  pods  and  seeds 
may  be  so  utilized.  By  far  the  greater  part  of  the  garden  peas 
grown  belong  to  the  shelling  group.  Commercial  varieties  of 
garden  peas  are  classified  on  the  basis  of  habit  of  growth — climb- 
ing, half-dwarf,  and  dwarf;  and  length  of  time  to  mature — early, 
medium,  and  late.  Peas  of  the  early  varieties  may  be  round  or 
wrinkled.  Most  of  the  medium  and  late  maturing  varieties 
belong  to  the  sugar  peas,  which  have  wrinkled  seeds  when 
mature.  Size  of  pod  is  another  important  classification  charac- 
ter. Ripened  pods  may  be  inflated  or  somewhat  constricted. 

Inheritance. — In  a  reciprocal  cross  of  the  varieties  Autocrat 
and  Bountiful,  it  has  been  suggested  (Keeble  and  Pellew,  1910) 
that  the  inheritance  of  the  character  tallness  involved  two  factor 
differences,  one  for  length  of  internode  and  one  for  thickness  of 
stem.  In  certain  crosses  White  (1918)  finds  the  inheritance  of 
stature  still  more  complicated.  Tall  varieties  (over  4.5  ft.)  are 
divided  into  three  groups  and  half-dwarfs  are  separated  into  two 
groups.  The  factorial  scheme  suggested  is  as  follows : 

FIG.  59. — Flower  structure  of  pea. 

1.  A  single  flower — a,  petals  of  calyx;  6,  side  view  of  corolla. 

2.  Front  view  of  fully  open  flower — a,  petal  of  calyx;  b,  standard;  c,  whig;  d, 
keel. 

3.  The   sexual  organs   removed  from  the  bud.      (Adapted  from  Muller.)     a, 
Filament;  b,  anther;  c,  style;  d,  stigma  hairs. 

4.  5.  Anthers. 

6.  Cross    section    ovary. 
9.  Longitudinal    section    ovary. 

Size:  1,  %n;  2,  Y§n\  3,  greatly  enlarged;  4,  5,  lOOn;  6,  greatly  enlarged; 
7,  8n;  8,  40n;  9,  40n. 


BREEDING  OF  VEGETABLES 


237 


d 


<f 


FIG.  59. 


238  BREEDING  CROP  PLANTS 

\  Le    =  long  internodes 
Length  of  internodes.  . .  f  . 

(  Lei  =  very  long  internodes 


Number  of  internodes. 


=  20-40  internodes 
i  =  40-60  internodes 
z  =  20-30  internodes 

Absences 

le     =  short  internodes 
t       =  10-20  internodes 

Le  is  the  height  factor  isolated  by  Mendel,  while  T  is  Keeble  and 
Pellew's  factor  for  thickness  of  stems  which  White  has  interpreted 
as  a  factor  for  internode  number  and  internode  length.  On  the 
factorial  basis  given,  the  phenotypic  condition  of  the  tails — of 
which  there  are  three  classes — would  be: 

1.  LeT    =  20-40  long  internodes. 

2.  Le  Ti  =  40-60  long  internodes. 

3.  LeiTz  =  20-30  very  long  internodes. 

The  phenotypic  nature  of  the  half  dwarfs  would  be : 

4.  Le  t  =  10-20  long  internodes. 

5.  le  T  =  20-40  short  internodes. 

True  dwarfs  would  represent  the  absence  of  both  dominant 
factors  Le  and  T  or  let.  With  the  same  material  that  Mendel 
used,  the  same  results  for  height  have  been  obtained. 

The  inheritance  of  time  of  flowering  involves  several  factors  as 
is  shown  by  complicated  F*  ratios  (Tschermak,  1916)  (Keeble 
and  Pellew,  1910).  Keeble  and  Pellew  found  linkage  between 
the  factors  for  thick  stems  and  late  maturity  and  likewise  between 
the  opposite  condition,  thin  stems  and  early  maturity. 

Vilmorin  (1910)  made  a  large  series  of  crosses  with  edible 
pod  races.  .  In  some  cases  both  FI  and  Fz  produced  only  plants 
with  edible  pods.  In  other  crosses  hard  inedible  pods  were  pro- 
duced in  FI  and  ratios  of  9  hard  to  7  edible  were  obtained  in  F2. 
Results  may  be  explained  by  the  supposition  that  hard  pod 
varieties  (development  of  parchment-like  tissue)  may  be  due  to  the 
presence  of  two  factors  (White,  1917)  PPVV.  Non-parchment 
varieties  may  have  either  the  f ormuladPPvv,  ppVV  or  ppvv.  White 
cites  earlier  workers  who  have  always  found  parchmented  pods 
to  be  inflated  and  non-parchmented  to  be  constricted.  Nohara 
(1918),  in  a  cross  between  a  Japanese  pea  and  a  French  variety, 
both  of  which  produced  soft  edible  pods,  obtained  a  ratio  of  9 
parchmented  to  7  non-parchmented  in  ^2,  Results  were  also 
explained  by  a  two-factor  hypothesis. 


BREEDING  OF  VEGETABLES 


239 


Tschermak  (1916)  has  compiled  a  brief  summary  of  the  mode 
of  inheritance  in  the  garden  pea  with  particular  reference  to 
economic  characters.  Table  LX  is  made  up  from  his  summary. 

TABLE  LX. — INHERITANCE  IN  THE  GARDEN  PEA 


Dominant 

Recessive 

Color  of  cotyledon1  

Yellow 

Green 

Form  of  cotyledon1  
Arrangement  of  seed  in  pod  . 
Character  of  pod  

Round 
Loose 
Full  and  smooth 

Wrinkled 
Crowded  close  together 
Constricted  and  wrinkled 

Ends  of  pod  
Color  of  unripe  pod  
Character  of  inflorescence.  .  . 
Tendrils 

Blunt 
Green 
Raceme 
Present 

Pointed 
Yellow 
Umbel 
Absent 

Leaf  surface  

Setting  of  pods  per  plant..  .  . 
Number  of  flowers  in  raceme 

Pubescent 

Imperfect  dominance 

Few 
3-5 

Smooth 

Mauy 
1-2 

1  Segregation  apparent  in  seeds  borne  by  Fi  plants. 

All  the  characters  listed  above  show,  as  a  rule,  simple  mono- 
hybrid  segregation  in  F%.  In  addition  to  these  the  following  char- 
acters are  intermediate  in  F\  and  give  complex  ratios  in  F% 
according  to  Tschermak: 

Long  pods  vs.  short  pods 
Broad  pods  vs.  narrow  pods 
Large  seed  vs.  small  seed. 

White  (1916)  has  done  much  to  increase  our  knowledge  of  the 
genetic  factors  and  their  interrelations.  Cotyledon  colors  were 
found  to  be  explained  satisfactorily  by  the  following  formulae: 


GGII  =  Dominant  yellow  varieties. 
ggii  =  Recessive  yellow  varieties. 
GGii  =  Green  varieties. 


(1) 
(2) 
(3) 


All  peas  have  yellow  pigment  in  the  cotyledon.  G  is  a  factor  for 
green  pigment  and  7  a  factor  which  causes  green  pigment  to  fade 
on  the  maturity  of  the  seed.  "Goldkonig"  was  the  only  variety 
containing  the  genetic  formula  ggii. 

Table  LXI  is  a  statement  of  the  genetic  factors  of  peas  as  deter- 
mined by  White  (1917),  who  made  a  careful  review  of  earlier 
studies  and  who  has  also  studied  a  number  of  new  crosses. 


240 


BREEDING  CROP  PLANTS 


TABLE  LXI. — LIST  OF  PISUM  FACTORS,  ALPHABETICALLY  ARRANGED,  AND 
THEIR  CORRESPONDING  CHARACTER  EXPRESSIONS 


Expression 


Salmon-pink  or  rose  flower  color.     With  CD  gives  reddish  leaf  axils. 
Purpling  factor  plus  A  gives  purple  flowers.     With  CD  plus  A  gives  purplish 

leaf  axils  and  stem  bases. 

Glaucous  foliage,  stems  and  pods  (with  W);  "bloom." 
Pods  with  blunt  apex. 
With  D  gives  leaf  axil  and  stem  color. 
With  C  gives  leaf  axil  and  stem  color. 
With  F  and  B  gives  purple  dotting  on  seed  coats:  in  the  absence  of  B  gives 

reddish  dots. 

Modifies  the  expression  of  (Lf)  toward  earlier  flowering. 
With  E  and  B  gives  purple  dotting  on  seed  coats;  in  the  absence  of  li  gives 

reddish  dots. 

Axillary  flowers,  round  stems,  regular  phyllotaxy. 
1  to  2  flowers  per  peduncle. 
Yellowish  green  to  grayish  brown  seed-coat  color  (weak  chromoseN  factor), 

brown  hilum. 
Green  cotyledon  pigment. 
Green  pod  color. 

Brightener  or  inhibitor  of  expression  of  (Gc.) 
Factor  which  causes  green  cotyledon  color  to  fade. 
With  (Gc)  gives  dark  brown  seed-coat  color. 
With  Li  gives  indent  peas. 
With  Li  (A)  gives  indent  or  dimpled  peas. 
Long  internodes;  with  T  gives  tall  plants. 
Primarily  responsible  for  late  flowering. 

Brown  or  maple  mottling  on  seed  coat;  or  "ghost  mottling"  in  absence  of  A. 
Violet  eye  on  seeds. 
Green  foliage,  stems,  and  pods. 
Inflated,  parchmented,  nonedible  pods  with  V. 
With  P->  gives  purple  pods. 
With  Pi  gives  purple  pods. 
Black-eyed  seed-coat  pattern. 
Round,  smooth  seeds  with  simple,  oval  starch  grains,  low  water  content 

and    with   excellent   powers   of   germination    under    unfavorable    weather 

conditions. 

Pods  with  seeds  separated  or  free. 
Tall,  robust  plants,  large  number  of  internodes. 
Leaves  with  tendrils. 
Dark  self-colored  purple  seed-coat. 
With  P  gives  parchmented.  smooth  pods. 
With  (B1)  gives  glaucous  foliage,  pods. 


The  presence  and  absence  of  these  thirty-five  factors  are  gene- 
tically responsible  for  seventy  or  more  differential  characters.  As 
is  noted  in  the  table,  there  is  a  modifying  effect  of  one  factor  upon 
another  in  certain  cases.  Studies  have  also  shown  that  certain 
environmental  conditions  may  modify  a  particular  inheritance  in 
such  a  way  that  the  true  genetic  nature  can  not  be  determined 
by  inspection.  This  is  an  instance  which  should  help  to  impress 
upon  the  student  the  necessity  of  the  controlled  breeding  test  as  a 


No. 

Factor 

j 

A 

2 

B  ' 

3 

(Bl) 

4 

(Bt) 

5 

C(A] 

6 

D 

7 

E[A] 

8 

(Ef) 

9 

F 

10 

(Fa) 

11 

(Fn) 

12 

(Gc)[A] 

13 

G 

14 

(Gp) 

15 

H 

16 

I 

17 

J 

18 

Li[A] 

19 

Lz 

20 

(Le) 

21 

(Lf) 

22 

M 

23 

N 

24 

O 

25 

P 

26 

Pi 

27 

P2 

28 

(PI) 

29 

R 

30 

S 

31 

T 

32 

(Tl) 

33 

U 

34 

V 

35 

W 

BREEDING  OF  VEGETABLES  241 

means  of  determining  the  genetic  nature  of  any  particular 
variety  or  strain. 

Factors  A,  C,  E,  (Gc)  and  L\  appear  absolutely  coupled  and 
may,  therefore,  be  considered  to  be  a  single  factor  with  several 
separate  expressions.  This  factor  gives  salmon-pink  or  rose  color 
to  the  flower,  and  to  the  leaf  axil,  and  to  the  stem  in  the  presence 
of  D;  purple  dotting  on  seed-coats  in  the  presence  of  F  and  B, 
with  reddish  dots  when  B  is  absent  and  F  is  present;  yellowish 
green  to  grayish  brown  seed-coat  color,  brown  hilum;  indent 
peas  in  the  presence  of  L2. 

The  results  of  examining  many  thousand  F2  generation  progeny 
indicate  that  factors  A,  B,  (Fa),  /,  (Le),  G,  and  R  are  indepen- 
dently inherited. 

Four  groups  of  linked  factors  were  found.  These,  according 
to  the  factorial  notation  used  by  White,  are: 

GROUP  PARTIALLY  LINKED  RATIO  OF  NON-CROSSOVERS 

TO  CROSSOVERS 

1  (Bl)S  8:1 

2  A(Lf)  7:1 

3  R(Tl)  63:1 

4  GO  Undecided 

BEANS 

Some  Classification  Characters. — The  species1  of  garden  beans 
most  commonly  grown  are  Phaseolus  vulgaris  and  P.  lunatus. 
The  former  is  divided,  from  the  standpoint  of  use  as  food,  into 
snap  and  shell  beans,  although  there  is  some  overlapping  in  these 
groups.  Shell  beans  are  sometimes  used  as  snap  beans  and  vice 
versa.  Time  required  to  mature,  habit  of  growth,  whether  climb- 
ing or  bush,  and  size  of  plant  are  characters  always  described  by 
commercial  seedsmen.  Length  of  bearing  period  is  also  an  im- 
portant character.  Commercial  growers  sometimes  desire  varie- 
ties which  may  be  harvested  in  a  few  pickings  but  for  the  home 
and  general  gardener,  a  variety  with  a  longer  bearing  period  is 
usually  preferred.  Size  and  shape  of  pod,  number  of  seeds  per 
pod  in  the  case  of  snap  beans,  quality  and  color  of  the  pod, 
are  used  in  classification;  with  snap  beans,  stringless,  fleshy, 
fine-grained  pods  are  most  desirable.  The  ease  with  which  dry 

1  For  a  discussion  of  the  classification  of  garden  beans  and  a  description 
of  varieties  see  TRACY,  W.  W.,  American  Varieties  of  Garden  Beans,  U.  S. 
D.  A.,  B.  P.  I.  Bull.  109,  173  pages,  1907;  JARVIS,  C.  D.,  American  Varieties 
of  Beans,  Cornell  Agr.  Exp.  Sta.  Bull  260:  149-255:  1908. 

16 


242  BREEDING  CROP  PLANTS 

shell  beans  may  be  thrashed  is  of  economic  importance.  In  this 
group,  color,  size,  and  shape  of  seeds  are  usually  included  in 
varietal  descriptions.  Both  productivity  and  disease  resistance 
may  differ  strikingly  in  different  varieties  of  beans. 

Inheritance. — Seed-coat  color  has  been  shown  by  Shaw  and 
Norton  (1918)  to  involve  several  factor  differences.  The  work 
was  carried  on  with  twenty-one  varieties  including  more  than 
40,000  plants.  Crosses  between  mottled  and  self-colored  varie- 
ties yielded  mottled  beans  in  FI  and  showed  3  :  1  ratios  in  F<^. 
Mottled  X  white  varieties  gave  mottled  in  FI,  and  in  Fz  the  ratio 
of  9  mottled  to  3  self-colored  to  4  white  usually  resulted.  It  was 
demonstrated  that  pigment  patterns  and  pigment  colors  were 
controlled  by  distinct  factors.  All  plants  with  white  or  eyed 
beans  bore  white  flowers  while  plants  with  mottled  or  self-colored 
beans  usually  bore  pink  flowers. 

The  inheritance  of  stature  in  beans,  as  in  peas,  is  in  some  crosses 
dependent  on  a  single  factor  difference  while  in  other  crosses 
several  factor  differences  are  involved.  Emerson  (1916)  has 
explained  the  result  of  crossing  a  tall  pole  (indeterminate  growth) 
bean  and  a  short  bush  (determinate  growth)  bean  or  a  short 
pole  bean  and  a  tall  bush  bean,  by  a  three-factor  hypothesis.  The 
following  values  to  be  added  to  an  initial  value  of  three  inter- 
nodes  were  assigned  to  the  factors :  Factor  A  either  homozygous 
or  heterozygous  added  10  internodes  approximately,  while  fac- 
tors B  and  C  each  added  two  internodes  when  homozygous  and 
one  when  heterozygous.  Results  were  explained  factorially 
as  follows: 

Parent  1 AABBCC  =  17  internodes  or  AAbbcc     =  13  internodes 

Parent  2 aabbcc         =    3  internodes  or  aaBBCC  =    7  internodes 

FI AaBbCc      =  15  internodes  or  AaBbCc    =  15  internodes 

Many  new  forms  would  naturally  be  produced  in  F%. 
Tschermak  (1916)  has  brought  together  and  summarized  the 

FIG.  60. — Flower  structure  of  bean. 

1.  Small  branch  showing — a,  developing  pod;  b,  c,  flowers  in  different  stages  of 
development. 

2.  Front  view  of  fully  opened  flower — a,  calyx;  b,  wing;  c,  standard;  d,  keel. 

3.  Enlarged  keel. 

4.  Keel  with  outer  part  broken  away  to  show — 6,  style;  c,  anther;  d,  undevel- 
oped pod;  e,  ovary. 

5.  6.  Longitudinal  and  cross  section  of  pod. 

7.  Enlarged  stigma  showing— a,  stigma  hairs. 

8.  Anther. 

Size:   1,  n;  2,  about  2w;  3  to  8,  greatly  enlarged. 


BREEDING  OF  VEGETABLES 


243 


FIG.  60. 


244 


BREEDING  CHOP  PLANTS 


data  on  the  inheritance  of  economic  characters  in  the  garden 
bean.     Table  LXII  is  made  up  from  his  summary. 

TABLE  LXII. — INHERITANCE  IN  THE  BEAN 


Contrasted  characters 

Fi  condition 

Fz  behavior 

Colored   X  white  (flowers)    

Colored 

3:1 

Green  X  yellow  (unripe  pods)  

Green 

3:1 

Non-constricted  X  constricted  (pods)  . 

Non-constricted 

3:1 

Round  X  flat  (pods)  

Round 

3:1 

"  Non-stringi  ness  "  X  "stringy" 

Intermediate  or  approach- 

Stringy pods  recessive 

(pods). 

ing  non-stringiness 

(1  out  of  4). 

Blunt  X  sharp  (pod  ends)  

Approaches  blunt 

Approaches  3:1. 

Broad  X  narrow  (pods)  

Approaches  broad 

Segregation  irregular. 

Long  X  short  (pods)  

Approaches  long 

Segregation  irregular. 

Cylindrical  X  spherical  (seeds)    .... 

Approaches  cylindrical 

Segregation      irregular 

with    spherical   seeds 

constant. 

Cylindrical  X  kidney-shaped  (seeds)  .  . 

Approaches  cylindrical 

Segregation     irregular 

with     kidney-shaped 

seeds  constant. 

Yellow  X  green  (cotyledons)  

Yellow  (apparent  on  crossed 

3  :  1     (segregation    ap- 

seed) 

parent  on  Fi  plants). 

The  inheritance  of  resistance  to  various  diseases  is  extremely 
important.  One  of  the  most  injurious  diseases  of  the  bean  is 
anthracnose  (Colletotrichum  lindemuthi anum) .  Barrus  (1918), 
as  the  result  of  an  extensive  study,  was  able  to  place  beans  in  four 
groups  with  respect  to  susceptibility  or  resistance  to  this  disease. 
Over  two  hundred  varieties  of  beans  commonly  grown,  besides 
many  others,  were  tested.  A  considerable  number  of  plants 
belonging  to  closely  related  genera  were  also  examined.  The 
cultures  of  anthracnose  used  for  inoculating  the  varieties  were 
obtained  from  widely  separated  geographical  areas.  By  study- 
ing the  reaction  of  the  various  cultures  to  each  bean  variety,  two 
strains  of  anthracnose,  alpha  and  beta,  were  discovered.  With 
respect  to  their  reaction  to  these  two  anthracnose  strains,  varie- 
ties of  beans  were  placed  in  four  groups: 

(ab)     Varieties  susceptible  to  both  strain  alpha  and  strain  beta. 
(aB)    Varieties  susceptible  to  strain  alpha  but  resistant  to  strain  beta. 
(Ab)    Varieties  susceptible  to  strain  beta  but  resistant  to  strain  alpha. 
(AB)  Varieties  showing  some  resistance  to  both  strains. 


The  most  resistant  variety  of  the  last  group  is  Wells  Red 
Kidney.     Results  of  crosses  between  varieties  whose  anthracnose 


BREEDING  OF  VEGETABLES  245 

reactions  are  known  indicate  (McRostie,  1919;  Burkholder, 
1918)  that  resistance  to  either  the  alpha  or  beta  strain  is  inherited 
as  a  simple  dominant,  involving  but  a  single  factor  difference. 
It  seems,  therefore,  very  easy  to  produce  resistant  varieties  to 
both  strains  by  crossing  and  selection  and  thus  to  combine  de- 
sirable economic  characters  and  anthracnose  resistance. 

McRostie  (1921)  has  recently  published  an  interesting  paper 
on  further  studies  of  disease  resistance  in  common  beans.  The 
more  extensive  results  obtained  bear  out  the  earlier  views  on 
the  mode  of  inheritance  of  resistance  to  bean  anthraxnose.  The 
studies  carried  out  show  that  bean  mosaic  susceptibility  is  in- 
herited. In  FI  there  was  a  partial  dominance  of  susceptibility 
over  resistance  and  in  Fz  a  segregation  which  indicated  a  two 
factor  hypothesis.  In  crosses  between  susceptible  and  resistant 
varieties  in  relation  to  the  dry  root  rot,  caused  by  the  fungus, 
Fusarium  martii  phaseoli  Burk.,  there  was  a  dominance  in  F\ 
of  susceptibility  and  a  segregation  in  F2  that  appeared  to  be  on 
a  9  : 7  basis.  In  nearly  all  cases  resistant  F2  plants  bred  true  to 
this  character  in  Fs.  Results  of  this  nature  show  the  great 
practical  importance  of  the  application  of  Mendelian  principles 
to  breeding  for  disease  resistance.  It  seems  very  likely  that  a 
large  part  of  our  serious  plant  diseases  will  be  controlled  even- 
tually by  the  production  of  disease  resistant  varieties. 

TOMATO 

Classification  Characters  and  Inheritance. — The  tomato  be- 
longs to  the  genus  Lycopersicum  of  which  there  are  several 
cultivated  species.  Tomatoes  are  classified  on  the  basis  of  vine 
habit,  either  standard  or  dwarf,  leaf  type,  period  of  maturity, 
size  and  color  of  fruits,  and  other  characters.  As  a  result  of 
breeding  experiments,  many  different  combinations  of  characters 
have  been  made.  Price  and  Drinkard  (1908)  were  among  the 
first  investigators  to  report  on  the  simple  Mendelian  behavior  of 
certain  tomato  characters.  Table  LXIII,  taken  from  similar 
ones  compiled  by  Tschermak  (1916)  and  Jones  (1917),  presents 
a  brief  summary  of  inheritance  in  the  tomato. 

Fruit  shape  is  dependent  on  several  factors  according  to 
Crane  (1915)  and  Groth  (1912,  1915).  Some  of  the  foliage 
characters  are  also  somewhat  complicated  in  their  inheritance 
(Groth,  1911).  The  inheritance  of  each  of  the  other  characters 
listed  in  the  table  is  dependent  on  single  factor  differences. 


246  BREEDING  CROP  PLANTS 

TABLE  LXIII. — INHERITANCE  OF  CHARACTERS  IN  THE  TOMATO 


Characters 

Dominant 

Recessive 

Fruit  shape  .... 

Spherical 

Pear-shaped 

Fruit  shape  
Loculation  of  ovary  
Endocarp  color  
Epicarp  color  
Fruit  surface  
Leaf  margin 

Roundish  conical 
Two-loculed 
Red 
Yellow 
Smooth 
Serrate  (normal  or  fine 

Roundish  compressed 
Many-loculed 
Yellow 
Colorless 
Pubescent 
Entire  (potato  or  coarse 

Leaf  type  

leaf) 
Pintpinellifolium  type 

leaf) 
Esculentum  type 

Leaf  color 

Green 

Yellow 

Inflorescence  type  
Vine     habit    and     leaf 
surface  
Height  of  plant 

Simple 

Standard,  smooth 
Tall  or  normal 

Compound 
Dwarf,  rugose 

The  Fz  segregation  ratio  is  3:1.  Jones  (1917)  has  pointed  out 
that  the  data  of  Hedrick  and  Booth  (1907)  and  Price  and  Drink- 
ard  (1908)  show  linkage  relations  between  the  factors  for  vine 
habit  and  fruit  shape  and  also  between  those  for  leaf  color  and 
loculation  of  ovary. 

Heterosis  in  the  F\  generation  of  certain  tomato  crosses 
and  its  commercial  possibilities  for  increased  production  have 
been  pointed  out  (Wellington,  1912;  Hayes  and  Jones,  1916). 
Groth,  of  the  New  Jersey  State  College  Experiment  Station, 
made  a  study  of  size  inheritance  in  the  tomato  fruit.  The  re- 
sults are  explained  by  what  the  author  (1914)  terms  "  Golden 
mean."  If  (a)  and  (6)  represent  the  respective  magnitudes  or 
volumes  of  size  characters  of  the  parents,  the  FI  is  represented  by 
\/ab  rather  than  (a  +  6)/2.  This  hypothesis  was  put  forward  as 
non-Mendelian  and  in  explanation  of  results  in  size  inheritance 
frequently  attributed  to  multiple  factors.  Emerson  (19146)  has 
shown  that  the  hypothesis  is  essentially  based  on  multiple  factors. 

PEPPERS 

Classification  Characters  and  Inheritance. — Garden  peppers 
which  are  commonly  grown  for  pickles  or  for  condiments  belong 
to  the  species  Capsicum  annuum.  From  the  standpoint  of  their 
utilization  as  food,  peppers  may  be  divided  roughly  into  two 
groups — hot  and  mild,  depending  on  flavor.  Mild  peppers  are 


BREEDING  OF  VEGETABLES 


247 


frequently  used  green  for  slicing  or  stuffing,  whereas  hot  peppers 
more  often  serve  as  a  condiment  in  spice  mixtures.  Number 
of  days  to  mature  is  usually  given  by  seeds-men  in  describing 
varieties.  Color,  size,  shape,  and  uniformity  of  fruit  are  other 
important  commercial  characters. 

A  limited  number  of  inheritance  studies  with  this  vegetable 
have  been  made.  Webber  (1911)  and  Ikeno  (1913)  report 
the  behavior  of  certain  characters  in  the  second  generation  after 
a  cross.  Below  is  given  a  tabular  summary  of  a  part  of  the  results 
obtained. 

TABLE  LXIV. — INHERITANCE  IN  THE  PEPPER 


Contrasted  characters 


Fi  condition 


Fz  behavior 


Violet  X  white  (flower).. .  . 


3:1 


Violet  (considerable 
variation  in  amount  of 
violet  coloring) 

Violet  flower  associated  with  violet  coloring  in  leaf-stem  and  ripe  fruit. 
White  flower  associated  with  green  leaf  and  stem  except  for  a  dark  spot 

near  attachment  of  petiole. 
Umbel    X  non-umbel    (in- 

Non-umbel 
Red 

Less   hairy  than    hairy 
parent 
Pungent 


florescence) 

Red  X  orange  (ripe  fruit).. 

Pubescence    X    non-pubes- 
cence (stems  and  leaves) 

Pungent  X  sweet  (fruit) . . . 


3:1 
3:1 

15  pubescent:! 

non-pubescent 

Approx.  3:1 


In  the  inheritance  of  size  of  leaf,  Webber  obtained  results  which 
clearly  indicated  that  several  factor  differences  were  involved 
and  a  like  result  was  obtained  by  both  Webber  and  Ikeno  with 
regard  to  size  of  fruit.  The  character  of  the  peduncle,  whether 
erect  or  recurved,  was  found  by  Ikeno  to  be  dependent  on  a 
single  factor  difference,  erect  being  the  recessive  condition  when 
the  fruit  had  ripened.  During  the  flowering  stage  and  early 
development  of  the  fruit  the  heterozygous  individuals  for  this 
character-pair  showed  dominance  for  the  erect  peduncle. 

Methods  of  Breeding  Self -Fertilized  Vegetables. — The  vege- 
tables discussed  above  together  with  others  which  are  naturally 
self-fertilized  or  which  will  produce  ample  viable  seed  when 
self ed  may  be  considered  as  a  single  group  from  a  breeding  stand- 
point. The  method  of  breeding  this  group  is  identical  with  that 
already  outlined  for  naturally  self-fertilized  crops  and  hence 


248  BREEDING  CROP  PLANTS 

need  not  be  repeated  here.  Yield,  quality,  and  disease  resistance 
are  the  three  most  important  economic  characters.  To  bring 
about  a  desirable  combination  of  these  characters,  both  selection 
and  hybridization  have  been  practiced. 

Selection  has  been  used  by  Edgerton  (1918),  of  the  Louisiana 
Agricultural  Experiment  Station,  to  isolate  tomatoes  resistant 
to  wilt  (Fusarium  lycopersici) .  The  improved  technic  followed 
is  worthy  of  consideration.  Seeds  of  a  particular  variety  were 
planted  in  soil  which  had  been  sterilized  previously  and  then 
inoculated  with  a  pure  culture  of  the  wilt-producing  organisms. 
When  seedlings  showed  wilt  infection  they  were  pulled  and  dis- 
carded. Only  plants  which  showed  resistance  were  transplanted 
to  the  field.  Tomatoes  had  grown  continuously  for  eight  or  ten 
years  on  this  field  and  it  was  known  to  be  heavily  infected  with 
the  wilt  fungus.  The  use  of  this  method  permits  a  smaller 
acreage  and  insures  the  contact  of  each  plant  with  the  wilt 
organism.  A  selection  made  from  a  row  of  Acme  grown  in  1909 
named  " Louisiana  Wilt-Resistant"  was  extremely  wilt  resistant 
but  possessed  other  characters  which  made  it  undesirable  for 
Louisiana  conditions.  Selections  from  the  progeny  of  crosses 
between  this  form  and  Earliana  showed  considerable  promise. 

Durst  (1918)  reported  the  result  of  five  years'  selection  for 
resistance  to  Fusarium  of  tomatoes.  Varieties  were  found  to 
differ  a  great  deal  in  their  resistance  and  unfortunately  the  most 
resistant  ones  produced  poor  fruit.  After  five  years,  some  of  the 
better  strains  stood  up  in  soil  which  proved  fatal  to  the  original 
varieties.  In  addition  to  disease  resistance,  the  selections  also 
showed  good  yielding  ability.  Of  seventy-four  lots  grown  one 
year  the  highest  fourteen  yields  were  produced  by  selected 
strains. 

Whether  selection  alone  or  hybridization  and  selection  together 
are  to  be  used  as  a  means  of  improving  a  crop  is  dependent  upon 
the  nature  of  the  material.  If  the  character  combination  is  not 
already  present,  the  only  practical  means  of  bringing  it  about 
is  crossing  followed  by  selection. 

Cross-Fertilized  Vegetables 

Crops  have  previously  been  classified  as  belonging  to  four 
groups  according  to  their  mode  of  reproduction.  Cross-fertilized 
vegetables  may  be  roughly  divided  into  three  main  divisions; 


BREEDING  OF  VEGETABLES  249 

1.  Those  which  are  normally  cross-pollinated  but  which  set 
seed  freely  on  selfing  and  show  no  evidence  of  sterility. 

2.  Those  which  are  wholly  or  partially  self-sterile. 

3.  Those   which   are    cross-fertilized  owing  to  the  dioecious 
condition. 

Much  more  study  of  the  mode  of  pollination  of  vegetables  is 
necessary  before  it  is  possible  accurately  to  classify  vegetables 
according  to  their  mode  of  reproduction.  The  crops  here 
considered  have  been  purposely  chosen  as  illustrations  of  breeding 
results  within  these  three  groups. 

RADISH 

Origin,  Inheritance,  and  Breeding. — The  cultivated  radish, 
Raphanus  sativus,  was  grown  by  the  ancient  Greeks  and  Romans. 
There  has  been  considerable  discussion  as  to  its  origin.  Some 
writers  have  thought  that  the  cultivated  form  with  its  fleshy 
root  arose  directly  from  R.  raphanistrum.  This  belief  was  ap- 
parently substantiated  by  experiments  in  which  the  wild  form 
was  grown  under  cultivation  and  after  several  years  cultivated 
radishes  were  obtained.  Riolle  (1914)  tested  this  hypothesis  by 
a  controlled  experiment.  The  wild  form  was  grown  under  culti- 
vation and  self-fertilized.  Three  years  of  selection  failed  to 
produce  roots  which  resembled  the  fleshy  roots  of  R.  sativus. 
On  the  other  hand,  when  the  wild  and  cultivated  forms  were  both 
grown  on  the  same  plot  and  seed  was  saved  from  the  wild  form, 
it  was  found  to  be  an  easy  matter,  after  three  years'  selection,  to 
obtain  roots  which  resembled  the  fleshy  roots  of  R.  sativus. 
These  results  were  believed  to  be  due  to  natural  crossing  of  the 
wild  and  cultivated  forms.  This  hypothesis  was  tested  by 
making  an  artificial  cross.  Segregation  for  root  condition  oc- 
curred in  FZ.  This  led  Riolle  to  conclude  that  former  experiments 
in  which  cultivated  radishes  were  obtained  from  the  wild 
through  selection  were  best  explained  through  natural  crossing. 

R.  sativus  roots  contain  sugar  while  wild  roots  contain  ho 
sugar.  FI  crosses  contain  less  sugar  than  the  cultivated  forms. 
The  presence  of  starch  in  the  root  of  the  wild  radish,  particularly 
in  the  bark,  is  a  character  which  separates  it  from  the  cultivated 
varieties.  This  proved  a  dominant  in  crosses.  Cultivated 
radishes  show  various  color  intensities.  Color  is  apparently 
inherited  in  much  the  same  manner  as  in  other  crops.  Individual 


250  BREEDING  CROP  PLANTS 

radish  plants  were  grown  under  cover  by  Riolle  and  self-fertilized 
seed  was  produced  in  abundance.  This  led  Riolle  to  suggest 
that  homozygous  strains  be  first  produced.  These  would  then 
furnish  material  for  accurate  inheritance  studies  as  well  as  be  of 
much  value  for  economic  breeding  purposes.  On  the  other  hand, 
Stout  (1920)  has  stated  that  there  is  considerable  self -sterility  in 
the  cultivated  radish.  Up  to  the  present,  mass  selection  has  been 
most  frequently  used  as  a  means  of  breeding  radishes  (Tschermak, 
1916). 

BEETS 

Inheritance  and  Breeding. — Both  garden  beets  and  sugar  beets 
belong  to  the  species  Beta  vulgaris.  Kajanus  (1913)  made  a 
study  of  the  inheritance  of  root  forms  in  mangels  and  sugar  beets. 
In  general,  the  F\  roots  were  intermediate  between  the  parental 
forms.  Sugar  beet  crosses  in  which  wedge-shaped  forms  were 
involved  proved  to  be  exceptions.  Wedge-shape  was  completely 
dominant  over  walnut-form  and  also  over  long,  somewhat  slender 
roots  (post-shape).  The  other  beet  shapes  studied  were  oval  and 
round.  Most  of  the  ratios  obtained  in  Fz  could  be  satisfactorily 
explained  on  the  basis  of  four  factors — two  involving  length  of 
root  and  two  concerned  with  form. 

A  marked  increase  in  the  sugar  content  of  the  sugar  beet 
was  produced  by  Vilmorin  through  the  application  of  the  progeny 
test  method  (see  page  119).  There  is  some  difference  of  opinion 
regarding  the  ease  of  producing  self -fertilized  seed.  Shaw  (1915) 
demonstrated  that  the  sugar  beet,  isolated  (two  miles  from  any 
other  beet  plants),  will  set  some  seed.  To  what  extent  self- 
sterility  is  a  factor  is  unknown.  The  production  of  homozygous 
forms  through  self-fertilization  would  seem  worth  trying  as  a 
means  of  obtaining  homozygous  material  for  breeding  studies. 
This  method  seems  a  logical  procedure  for  all  vegetables  which 
are  naturally  cross-fertilized  but  which  also  set  seed  freely  under 
conditions  of  self-fertilization. 

Mass  selection  is  often  used  in  breeding  beets.  Only  those 
roots  which  come  up  to  an  adopted  standard  are  stored  over 
winter  and  set  out  the  following  spring  to  become  the  seed- 
producing  plants.  Carrots  and  parsnips,  when  bred  by  mass 
selection,  are  handled  in  a  similar  manner.  Although  varieties 
of  any  one  of  the  crops,  beets,  carrots,  or  parsnips,  freely  inter- 
cross, there  is  no  crossing  between  the  three  different  kinds  of 


BREEDING  OF  VEGETABLES  251 

vegetables  (Malte  and  Macoun,  1915).     This  fact  may  be  utilized 
in  making  planting  plans. 

CULTIVATED  VEGETABLES  OF  THE  GENUS  BRASSICA 

Cabbage  and  several  other  vegetables  such  as  cauliflower, 
brussels  sprouts,  kohl-rabi,  and  rutabagas,  belong  to  the  genus 
Brassica.  Few  inheritance  studies  have  been  made  with  this 
group  of  vegetables.  Cabbage  has  received  more  attention  from 
a  breeding  standpoint  than  the  others. 

Inheritance. — The  evidence  so  far  accumulated  indicates 
that  cabbage  belongs  to  the  cross-fertilization  obligatory  group. 
Price  (1911-1912)  and  Jones  and  Oilman  (1915)  were  not 
able  to  produce  self-fertilized  seed  under  a  bag.  Tschermak 
(1916)  maintains  that  many  of  the  kinds  of  vegetables  belonging 
to  the  cabbage  group  freely  intercross  when  in  close  proximity 
at  blooming  time.  The  above  facts  are  fundamental  and  show 
the  method  of  breeding  which  must  be  used.  They  may  also  aid 
in  explaining  some  unusual  inheritance  results. 

Price  crossed  varieties  of  crinkled-leaf  and  smooth-leaf  cabbage, 
obtaining  dominance  for  crinkled  leaf  in  FI  with  no  segregation 
of  this  character  in  F2  ,i.e.,  all  plants  (419)  had  crinkled  leaves. 
With  respect  to  size,  shape,  and  solidity  of  heads,  color  of  foliage, 
and  length  and  thickness  of  stem,  considerable  more  variability 
was  obtained  in  F2  than  in  FI.  In  a  cross  between  a  crinkled- 
leaf  cabbage  and  a  cauliflower,  the  thick,  leathery  leaf  of  the 
latter  was  dominant  in  FI  and  was  the  only  apparent  leaf  char- 
acteristic in  P2.  Head  cabbage  crossed  with  headless  produced 
nothing  but  headed  forms  both  in  the  FI  and  F2  generations. 
As  to  type  of  head,  the  cabbage  or  leafy  form  was  found  to  be 
dominant  over  the  type  of  head  of  the  cauliflower.  In  F2  the 
cabbage  head  form  was  maintained  without  apparent  segrega- 
tion. Crosses  between  cabbage  and  brussels  sprouts  gave  FI 
and  F2  generations  identical  with  respect  to  habit  of  growth,  i.e., 
all  were  determinate.  Axillary  buds  were  more  common  in  the 
hybrids  than  in  ordinary  cabbage.  The  thick  stem  of  kohl- 
rabi was  found  to  be  dominant  in  a  kohl-rabi-cabbage  cross 
and  a  limited  number  of  F2  individuals  showed  no  segregation 
of  this  character. 

Button  (1908)  crossed  reciprocally  kohl-rabi  and  Drumhead 
cabbage,  obtaining,  in  F^  3  non-kohl-rabi  plants  to  1  resembling 


252  BREEDING  CROP  PLANTS 

kohl-rabi.  The  parental  forms  did  not  appear  in  the  F2  genera- 
tion. Drumhead  cabbage  crossed  with  Thousand-headed  kale 
produced  204  plants  in  F2.  Of  these,  176  resembled  a  dwarf 
type  of  Thousand-headed  kale  with  leaves  broader  than  usual 
and  fewer  branches;  26  resembled  cabbage;  and  two  plants 
were  much  like  brussels  sprouts. 

The  difficulty  of  a  study  of  inheritance  in  the  Brassica  genus 
arises  from  the  heterozygous  condition  of  many  forms  and  the 
self-sterile  condition.  Before  the  results  are  accepted  as  ex- 
amples of  non  Mendelian  behavior,  a  criticial  study  in  which  all 
facts  are  considered  should  be  made.  In  cabbage  there  is  appar- 
ently a  complicated  inheritance.  The  above  results  are  satis- 
factorily explained  on  a  multiple-factor  hypothesis.  In  crossing 
heterozygous  forms,  the  FI  generation  may  be  as  variable  as  the 
F2.  In  the  inheritance  of  any  particular  character,  the  number  of 
factor  differences  may  be  so  large  as  to  make  the  appearance  of 
parental  forms  improbable  in  a  small  Fz  generation. 

Breeding. — The  breeding  of  cabbage  resistant  to  yellows  (Fusa- 
rium  conglutinansWollenw.)  at  the  Wisconsin  Experiment  Station 
(Jones  and  Oilman,  1915)  is  of  great  economic  importance. 
Less  than  a  decade  ago,  truck  farmers  in  certain  sections  of  Wis- 
consin were  so  discouraged  from  the  ravages  of  yellows  that  they 
were  about  to  abandon  cabbage  growing.  The  method  of  pro- 
ducing resistant  cabbage  strains  may  be  briefly  summarized. 
It  had  been  noticed  that  there  were  usually  a  few  plants  which 
escaped  the  disease  in  a  field  where  nearly  all  plants  were  badly 
infected  with  the  organism.  These  apparently  resistant  plants 
were  selected  on  the  basis  of  type.  After  storing  over  winter, 
all  that  were  of  the  same  general  type  were  planted  together 
and  were  far  enough  removed  from  any  other  similar  planting 
to  insure  against  contamination  by  foreign  pollen.  Selfed 
seed  was  not  obtained  but  most  plants  not  bagged  set  seed 
abundantly.  Some  plants  were  eliminated  because  of  low  seed 
production.  Progeny  of  the  retained  plants  were  grown  sepa- 
rately and  their  resistance  to  yellows  was  tested.  In  this  way 
several  strains  of  cabbage  highly  resistant  to  yellows  have  been 
produced.  Further  studies  have  been  reported  and  numerous 
resistant  varieties  have  been  produced  (Jones  et  al,  1920).  The 
writers  emphasize  the  fact  that  resistance  is  not  absolute  and 
that  environmental  factors  influence  very  markedly  the  develop- 
ment of  the  disease.  They  state,  however,  that: 


BREEDING  OF  VEGETABLES  253 

"By  following  the  proper  methods  any  skillful  cabbage  grower 
who  has  Fusarium  sick  soil  may  either  undertake  with  reason- 
able confidence  to  develop  a  resistant  strain  of  his  own,  or  having 
secured  one  of  these  resistant  strains  he  can  maintain  its  resistance 
and  produce  his  own  seed." 

ASPARAGUS 

Asparagus  (Asparagus  officinalis)  is  dioecious  in  habit  of  flow- 
ering altho  hermaphrodite  plants  have  been  discovered  (Norton, 
1911-1912).  With  this  vegetable,  cross-pollination  is  usually 
necessary  for  seed  production. 

Rust-Resistant  Asparagus. — The  fungus,  Puccinia  asparagi, 
has  occasioned  a  great  deal  of  alarm  among  commercial  asparagus 
growers,  particularly  those  of  the  eastern  United  States.  This 
rust  differs  from  that  occurring  on  the  small  grains  in  that  all 
stages  of  the  rust  occur  on  the  asparagus  plant.  At  the  invita- 
tion of  Massachusetts  growers,  the  United  States  Department  of 
Agriculture  in  cooperation  with  the  Massachusetts  Agricultural 
Experiment  Station  undertook  to  produce  a  resistant  variety. 
Norton  (1911-1912,  1913)  has  reported  on  this  investigation. 
Because  of  the  dioecious  habit  of  asparagus  it  was  necessary  to 
select  two  kinds  of  plants — male  and  female.  Selections  were 
based  on  rust  resistance,  i.e.,  only  plants  which  showed  a  high 
degree  of  resistance  were  chosen.  In  1909  the  first  test  of  the 
transmission  of  relative  rust  resistance  was  made.  Twelve  lots 
saved  from  as  many  plants  showing  various  degrees  of  rust  re- 
sistance were  planted  in  duplicate  in  short  rows.  After  the 
young  shoots  appeared  they  were  dusted  several  times  with  fresh 
uredospores.  Later  in  the  season  observations  were  made  on 
the  degree  of  infection.  The  results  are  given  in  Table  LXV 
(Norton,  1913). 

Table  LXV  shows  clearly  that  rust  resistance  is  inherited. 
Various  artificial  crosses  were  made  between  forms  showing  rust 
resistance.  The  progeny  of  some  of  these  crosses  proved  highly 
resistant  and  in  some  cases  were  more  resistant  than  the  parents. 
By  this  method  several  strains  of  asparagus  with  a  high  degree  of 
resistance  have  been  produced.  In  the  production  of  a  new  form 
a  male  plant  obtained  in  1910  from  a  lot  of  New  American  of  un- 
known origin  proved  of  marked  ability  in  transmitting  vigor  and 
rust  resistance  to  the  progeny.  The  female  plants  known  as 
Mary  and'  Martha  were  selected  from  the  variety  Reading 


254 


BREEDING  CROP  PLANTS 


TABLE  LXV. — TRANSMISSION  OF  RUST  RESISTANCE  IN  ASPARAGUS 


Row 

Source  of  seed 

Type  of  plant 

Rank  of  seedlings  in 
resistance 

First 
lot 

Second 
lot 

Average 

1 

2 
3 

4 
5 
6 
7 
8 
9 
10 
11 
12 

A  1-6 

A3-61 
A4-7 
,4  4-17 
A7-5 
A7-15 
47-25 
524-27 
524-28 
Old  field 
Old  field 
Frank  Wheeler 
old  bed 

Badly  rusted,  near  rusty 
bed 

7 
6 
3 
10 
4 
2 
5 
11 
9 
12 
8' 
1 

9 
5 

7 
8 
3 
4 
2 
10 
11 
12 
6 
1 

8.0 
5.5 
5.0 
9.0 
3.5 
3.0 
3.5 
10.5 
10.0 
12.0 
7.0 
1.0 

Very  resistant  female  
Resistance  good       .... 

Resistance  fair  
Resistance  good  

Resistance  good  

Resistance  good       .  .    . 

Very  rusty 

Very  rusty  

Rusty  
Resistant 

Best  resistant  female  

Giant.  Two  or  three  other  females  have  been  selected  and  the 
crossed  seed  obtained  from  these  selected  plants  has  been  distrib- 
uted under  the  name  Washington  asparagus  (Norton,  1919). 
Some  of  these  strains  are  now  being  offered  for  sale  by  commercial 
seedsmen. 

Norton  suggests  the  following  method  for  breeding  asparagus; 
After  two  mated  plants  have  proved  their  value  by  the  progeny 
test,  they  should  be  dug  up  and  propagated  by  crown  division. 
These  clones  are  isolated  together  and  retained  exclusively  as 
breeding  stock.  Isolation  may  be  accomplished  by  a  fine-meshed 
cage  to  prevent  the  entrance  of  bees  or  by  planting  at  a  safe 
distance  from  other  beds  of  asparagus.  Producing  seed  in  a 
greenhouse  by  hand  pollination  has  also  been  found  successful. 

ECONOMIC  CUCURBITACE^ 

Introduction  and  Classification. — The  family  Cucurbitacece 
is  of  considerable  historical  interest.  Sageret  (1826)  and  Naudin 
(1856,  1859a,  18596),  two  pre-Mendelian  workers,  made  extensive 
hybridization  studies  with  some  species  belonging  to  this  family. 
Naudin  made  a  species  classification  on  the  basis  of  genetic 
behavior  which  is  accepted  at  the  present  time.  All  the  forms 
which  cross  readily  were  placed  in  the  same  species  group. 


BREEDING  OF  VEGETABLES 


255 


Cucumis  sativus — Cucumber 

Cucumis  melo  —  Muskmelon,  cantaloupe 

Cucurbita  pepo  — Pumpkin,  gourd,  summer  squash,  and 

varieties    of    winter    squash.     Peduncle 

hard  and  ridged. 
Cucurbita  maxima — Large  field  squash  and  winter  squash. 

Peduncle  soft  and  fleshy. 
Cucurbita    moschata — Squash.     Little    grown    in    United 

States.     Peduncle  much  enlarged  where 

attached  to  fruit. 
Citrullus  vulgaris — Watermelon,  citron. 


FIG.  61. — Structure  of  flowers  of  squash. 

1.  Female  flower — a,  corolla;  b,  calyx;  c,  fruit. 

2.  Male  flower. 

3.  Male  flower  with  calyx  and  corolla  removed. 

4.  Female  flower  with  calyx  and  corolla  removed  showing — a,  stigma;  6,  style; 
c,  point  of  attachment  of  calyx  and  corolla;  d,  undeveloped  fruit. 

5.  6.  Longitudinal  and  cross  sections  of  fruit. 
Size:   1,  2,   Y±n;     3,  4,  >£n. 

Cummings  (1904)  experienced  no  great  difficulty  in  crossing 
Golden  Custard  ( 9)  with  Crookneck  (cf),  varieties  of  squashes 
belonging  to  C.  pepo.  The  reciprocal  cross  proved  difficult,  only 
five  out  of  284  pollinations  producing  fruit  with  viable  seed.  A 
histological  examination  revealed  the  fact  that  the  male  generative 


256  BREEDING  CROP  PLANTS 

nucleus  of  Custard  penetrated  the  ovary  of  Crookneck  and  took 
up  a  position  which,  in  many  cases,  was  in  close  proximity  to  the 
egg  cell  but  for  some  reason  fusion  did  not  occur  in  most  cases. 

Bailey  (1890),  as  the  result  of  many  artificial  pollinations, 
concludes  "that  the  field  pumpkins  and  the  summer  and  fall 
types  of  bush  squashes  (C.  pepo)  do  not  cross  with  the  running 
squashes  of  the  Hubbard,  Marblehead,  Boston  Marrow,  turban, 
and  mammoth  types  (C.  maxima)."  In  the  Cyclopedia  of 
American  Horticulture,  Bailey  (1900)  states  that  C.  moschata 
and  C.  pepo  may  be  crossed  artificially  but  it  is  doubtful  if 
they  cross  naturally.  Cucurbitacece  in  general  are  monoecious 
and  largely  dependent  on  insects  for  pollination. 

Immediate  Effect  of  Pollination. — There  is  a  popular  belief 
widely  disseminated  that  pumpkins  and  watermelons  should 
not  be  grown  in  close  proximity  to  one  another  because  of  the 
immediate  effect  of  cross-pollination.  A  similar  belief  exists 
with  regard  to  cucumbers  and  muskmelons.  Evidence  accumu- 
lated by  various  plant  breeders  shows  that  this  idea  is  not  founded 
on  fact.  The  work  of  Bailey  at  Cornell  and  Pammel  at  Iowa 
may  be  cited.  The  former  (1890)  was  unable  to  find  any  immedi- 
ate effect  of  cross-pollination  between  varieties  of  C.  pepo  and 
likewise  between  varieties  of  C.  maxima.  Bailey  not  only  was 
unable  to  demonstrate  any  immediate  effect  of  pollen  in  varieties 
which  could  be  crossed  but  he  was  even  unable  to  produce  crosses 
between  cucumbers  and  muskmelons.  Ninety-seven  flowers  of 
several  varieties  of  melons  were  pollinated  with  different  varieties 
of  cucumbers.  Not  a  single  fruit  set.  Twenty-five  reciprocal 
pollinations  were  also  made.  One  fruit  developed  but  produced 
no  seed.  The  setting  of  parthenocarpic  fruit  without  fertilization 
is  not  an  infrequent  occurrence  in  cucumbers.  Pammel  (1892), 
in  an  intermingled  planting  of  varieties  of  each  of  the  following 
species,  Citruttus  vulgaris,  Cucumis  melo,  Cucurbita  maxima, 
Cucumis  sativus,  and  Cucurbita  pepo  provided  excellent  facilities 
for  inter-specific  pollinations.  Neither  the  watermelons  nor  the 
muskmelons  showed  contamination.  Some  hand  pollinations  be- 
tween species  were  made,  but  no  cross-fertilization  was  obtained. 

The  variability  in  flavor  of  commercial  varieties  of  melons  is 
undoubtedly  partly  responsible  for  the  erroneous  belief  that 
they  may  be  contaminated  by  other  species  of  cucurbits  grow- 
ing in  close  proximity.  At  the  Connecticut  Station  an  extensive 
varietal  test  was  made.  Most  of  the  varieties  were  of 
very  inferior  quality  even  though  they  were  exposed  only  to 


BREEDING  OF  VEGETABLES 


257 


rnuskmelon  pollen.     Even  if  crossing  occurred,  there  is  no  con- 
clusive evidence  that  xenia  would  result. 

CUCUMBER 

Wellington,  (1913)  studied  the  inheritance  of  the  following  char- 
acters: color,  size,  number  of  spines,  smooth  or  rough  skin,  and 
obtained  ratios  indicating  monohybrid  segregation.  Smooth  skin 
and  small  spines,  few  in  number,  appear  to  be  linked.  Heterosis 
shown  by  increased  number  or  size  of  fruit,  has  been  observed 
in  the  F\  of  certain  cucumber  crosses  (Hayes  and  Jones,  1916). 
The  Fi  of  a  cross  (Reeves,  1918)  between  American  type  (20  per 
cent  parthenocarpic)  and  English  type  (normally  parthenocarpic) 
showed  20  per  cent  parthenocarpy. 

MUSKMELON 

Lumsden  (1914),  of  the  New  Hampshire  Agricultural  Experi- 
ment Station,  has  made  rather  extensive  studies  of  inheritance 
in  the  muskmelon.  The  following  tabular  statement  gives  a 
summary  of  his  work: 

TABLE  LXV1. — INHERITANCE  IN  THE  MUSKMELON  IN  A  CROSS  BETWEEN 
THE  VARIETIES  SUTTON'S  SUPERLATIVE  AND  DELICES  DE  LA  TABLE 


Characters 

No.  of 
Fi 

plants 

„ 

No.  of 
Fz 
plants 

Fi  ratio 

Color  of  skin     .    .        .    . 

Yellow  *  green 

Yellow  X  green  

1 

Yellow 

79 

2  76:1 

Form  of  fruit                 .    . 

Round  •  elliptical 

Round  X  elliptical  

1 

Round 

79 

2.76:1 

Ribbing  

5-45        46-100 
per  cent.       per  cent, 
ribbing  •  ribbing 

Ribbed  X  non-ribbed.  .  . 

1 

Ribbed 

79 

1:1.82 

Netting  

5-45       46-100 
per  cent.       per  cent. 
Netting  •  netting 

Netted  X  smooth  

1 

Netted 

79 

1  :  1  .  63 

•Size  of  seed  
Large  X  small  

1 

Large 

79 

Large  :  small 
2.95:1 

Size  of  fruit 

Large  *  small 

Large  X  small  

1 

Large 

79 

2.59:1 

17 


258  BREEDING  CROP  PLANTS 

These  data  show  that  all  the  characters  studied  segregated 
in  the  second  generation.  There  is  some  indication  that  the 
contrasted  characters  in  color  of  skin,  size  of  seed,  size  of  fruit, 
and  form  of  fruit  are  each  separated  by  a  single  main  factor 
difference.  In  a  cross  between  varieties  producing  round  and 
elliptical  fruits  respectively  the  F\  fruit  was  recorded  as  round, 
while  the  Fz  gave  a  ratio  of  2.76  round  to  1  elliptical.  The  other 
two  characters,  netting  and  ribbing,  indicate  more  complex  in- 
heritance. Delices  de  la  table  (cf)  has  deep  ribbing  and  no 
netting;  Sutton's  Superlative  (  9  )  has  no  ribbing  and  pronounced 
netting.  The  F2  generation  showed  a  variation  of  from  5  to  100 
per  cent,  with  respect  to  each  character. 

SQUASHES  AND  GOURDS 

Emerson  (1910),  while  at  the  Nebraska  Experiment  Station, 
made  a  study  of  size  inheritance  in  a  cross  between  Yellow 
Crookneck  and  White  Scallop  summer  squashes.  He  found 
that  length  of  neck  and  diameter  of  bowl  were  intermediate 
between  the  parents  in  FI.  The  second  generation  showed  a 
complete  series  of  dimensions  and  shapes  from  one  parent  to 
the  other.  The  same  investigator  crossed  Striped  Spoon  gourd 
with  Filipino  Horned  gourd.  Results  similar  to  those  of  the 
squash  cross  were  obtained. 

WATERMELON 

One  of  the  most  serious  handicaps  to  the  production  of  water- 
melons in  the  Southern  States  is  the  presence  of  wilt,  due  to  an 
organism,  Fusarium  niveum.  "Citron"  or  "stock  melon,"  so- 
called  locally,  is  a  non-edible  variety  of  Citrullus  vulgaris  resistant 
to  wilt.  Orton  (1911)  conceived  the  idea  of  crossing  this  form 
with  edible  forms.  He  hybridized  Eden,  a  good  quality  melon, 
with  citron.  The  FI  was  very  vigorous  and  of  intermediate  type. 
Between  three  and  four  thousand  Fz  plants  were  grown  and 
ten  fruits  selected  on  the  basis  of  resistance  and  quality.  After 
selecting  the  resultant  progeny  for  several  years  the  variety 
Conqueror  was  isolated.  It  is  disease  resistant,  has  a  tough 
rind,  and  does  not  sunburn  easily.  The  flesh  is  juicy  and  of 
good  quality,  although  not  equal  to  the  finest.  These  studies 
were  made  in  South  Carolina.  It  was  found  that  Conqueror 


BREEDING  OF  VEGETABLES 


259 


retained  its  resistance  when  grown  in  Iowa  but  seemed  to  lose  it 
when  grown  in  Oregon,  on  the  Pacific  Coast.  No  very  satisfac- 
tory explanation  has  been  offered  for  this  phenomenon.  It  is 
possible  that  a  similar  condition  exists  with  flax  wilt. 


FIG.  62. — A  strain  of  Hubbard  squash  isolated  by  self-fertilization  which  is 
comparatively  uniform  for  the  production  of  large  fruits  of  uniform  shape. 
Minnesota  Exp.  Sta.  (Courtesy  of  Bushnell,) 

Flax  strains  resistant  to  wilt  seem  to  lose  their  resistance  when 
grown  for  a  few  years  in  wilt-free  soil. 

Breeding  Cucurbitaceae. — Each  botanical  species  of  this 
family  in  most  cases  constitutes  a  freelv  inter-crossing  group  of 


FIG.  63. — A  small  fruited  strain  isolated  from  a  commercial  variety  of  Hubbard 
squash  by  self-fertilization.      Minnesota  Exp.  Sta.     (  Courtesy  of  Bushnell.) 

varieties.  The  monoecious  character  of  the  plant  encourages 
cross-fertilization.  In  spite  of  these  facts  the  authors  believe 
that  in  some  cases  progress  may  be  made  by  breeding  methods 


260  BREEDING  CROP  PLANTS 

recommended  for  self-fertilized  crops  or  more  specifically 
for  crops  which  yield  ample  seed  on  selfing.  When  such  a  plan 
is  adopted  for  naturally  cross-fertilized  crops  it  becomes  necessary 
to  insure  selfing  by  artificial  means.  By  reducing  ordinary  varieties 
to  pure  lines,  a  much  more  exhaustive  study  of  the  material  at  hand 
may  be  made,  and  on  the  basis  of  this  study  desirable  combina- 
tions affected  by  hybridization  or  pure  lines  of  commerical  value 
may  be  isolated.  The  method  which  is  adopted  after  the  isola- 
tion of  homozygous  lines  through  self-fertilization  will  depend 
on  the  degree  in  which  vigor  is  lost  as  a  result  of  selfing.  That 
homozygous  lines  may  be  isolated  in  squashes  is  a  demonstrated 
fact,  the  result  of  three  years'  study  as  carried  on  by  John  Bushnell, 
of  the  Minnesota  Station.  Some  lines  which  are  comparatively 
uniform  appear  vigorous  while  others  are  less  vigorous.  Types 
for  markedly  different  characters  which  are  relatively  uniform 
have  been  isolated. 


CHAPTER  XVIII 
FRUIT  BREEDING 

The  improvement  of  fruit  crops  offers  an  interesting  field  of 
study  for  the  trained  investigator.  Many  fruits  are  in  a  complex 
heterozygous  condition.  For  this  reason  and  because  fruits  are 
propagated  by  asexual  methods  Mendel's  law  does  not  have  here 
the  same  value  as  for  the  breeder  of  self -fertilized  crops.  There 
are  also  many  fruit  crops  which  are  totally  self -sterile  so  that 
cross-pollination,  either  natural  or  artificial,  is  essential  to  the 
production  of  fruit.  Unlike  an  annual  crop  the  individual  fruit 
tree  often  takes  many  years  to  grow  before  fruiting.  For  these 
reasons  methods  of  handling  are  often  of  much  greater  importance 
than  methods  of  breeding.  It  is,  therefore,  of  utmost  importance 
that  the  student  first  make  an  intensive  study  of  the  botanical 
relatives,  methods  of  culture,  varieties,  and  environmental 
necessities  of  the  crop  before  undertaking  breeding  operations. 

ORIGIN  AND  ANTIQUITY  OF  SOME  FRUITS1 

Wild  fruits  without  doubt  played  an  important  role  in  the 
food  supply  of  primitive  man.  As  the  art  of  agriculture  came 
to  be  developed  because  of  the  necessity  of  obtaining  enough 
food  to  supply  the  increasing  human  population,  the  fruit  crops 
were  gradually  introduced  into  cultivation.  Some  of  our  most 
prized  fruits,  as  the  apple,  grape,  and  plum,  have  been  cultivated 
since  earliest  times;  while  others,  as  the  strawberry,  black  rasp- 
berry, and  blackberry,  have  been  brought  under  cultivation 
since  America  was  discovered. 

The  wild  species  from  which  our  fruits  have  been  developed 
may  still  be  found  today.  Wild  plums  may  be  found  in  nearly 
every  state  of  the  United  States,  while  in  central  and  northern 
Asia  the  wild  relatives  of  apples,  pears,  apricots,  cherries,  and 
plums  are  of  frequent  occurrence. 

The  wild  crabs  are  found  in  abundance,  in  both  the  Eastern 
and  the  Western  Hemispheres.  As  the  cultivated  European 

1  A  paper  by  WHITE  (1916)  has  been  used  very  freely  in  this  discussion. 

261 


262  BREEDING  CROP  PLANTS 

varieties  gave  good  results  when  introduced  into  the  United 
States,  the  breeding  of  apples  has  not  been  seriously  undertaken 
until  comparatively  recent  times.  The  cultivated  varieties  are 
very  numerous.  Our  pears  were  developed  from  two  very  dif- 
ferent wild  species,  Pyrus  communis,  the  wild  pear  of  western 
Asia  and  Europe  and  the  hard,  gritty  sand  pear  of  northern  China. 
P.  communis  is  the  source  of  our  eating  pears,  such  as  the 
Bartlett,  while  inter-species  crosses  furnished  our  cooking  and 
winter  pears. 

Peaches  were  first  developed  in  China.  When  one  compares 
the  little  hard,  bitter  wild  peach  of  China  and  our  cultivated 
varieties  the  results  of  early  breeding  are  strikingly  illustrated. 

There  are  three  groups  which  are  commonly  accepted  as  the 
ancestral  forms  of  our  cultivated  plums:  (1)  The  thorny  wild 
European  species  which  produces  dark  purple  fruits  about  the 
size  of  a  pea.  These  are  the  source  of  our  prune  varieties.  (2) 
North  American  native  wild  plums  which  have  a  very  juicy 
flesh  without  much  meat.  Several  species  are  recognized 
(Wight,  1915).  (3)  A  Chinese-Japanese  wild  species.  Many  of 
the  cultivated  varieties  of  plums  are  largely  of  hybrid  origin. 

There  are  over  120  wild  species  of  cherries  which  are  native  to 
Asia  and  from  200  to  1,500  wild  species  of  raspberries  and  black- 
berries. The  variation  in  type  of  the  wild  red  raspberries  of 
New  England  is  a  good  illustration  of  a  wide  diversity  of  forms. 
Some  of  these  are  probably  results  of  crosses  with  escaped  culti- 
vated varieties.  Natural  hybridization  certainly  played  a  large 
part  in  the  evolution  of  such  fruits  and  the  selection  of  promis- 
ing wild  seedlings  furnished  the  major  part  of  our  cultivated 
varieties. 

Fletcher  (1916)  has  described  1879  varieties  of  strawberries 
which  originated  in  North  America  and  26  European  varieties 
which  have  attained  prominence  in  this  country.  The  straw- 
berry is  largely  a  hybrid  product  of  four  or  more  species. 

The  citrus  fruits  are  all  of  Asiatic  origin.  Present  cultivated 
varieties  have  for  the  most  part  been  produced  during  the  last 
100  years.  The  grapefruit  industry  of  the  United  States  has 
been  developed  in  the  last  25  years.  This  fruit,  which  is  a  native 
of  islands  lying  to  the  south  of  Asia,  was  introduced  into  the  West 
Indies  early  in  the  eighteenth  century  and  more  recently  from 
the  West  Indies  into  Florida.  Table  LXVI1,  which  is  part  of  a  table 
published  by  White  (1916),  is  a  summary  statement  of  the  source 


FRUIT  BREEDING 


263 


and  the  length  of  time  under  cultivation  of  some  of  our  most 
highly  prized  fruit  crops. 

TABLE  LXVII. — ORIGIN,  PROBABLE  LENGTH  OF  TIME  OF  CULTIVATION,  AND 
COMMENTS  ON  SOME  CULTIVATED  FRUITS  (AFTER  WHITE,  1916) 


Name 

Date 

Origin 

Remarks 

Apple  

Apricot  
Blackberry 

•    A 

A 

F 

E.  Europe,  W.  Asia 

Central  Asia,  China 
United  States 

Very  different  type  common  to 
China. 
Wild  species  variable. 
Wild  species  verv  variable. 

Blueberry  .  . 

F 

E.  and  N.  North  America. 

Four    species,    often    confused 

Cranberry  
Currant,  red  

Cherry,  sour  
Cherry,  sweet  
Grape,  Old  World  .  . 
Grape,  New  World  .  . 
Gooseberry  

F 
C 

B 
B 
A 

F 
C 

E.  and  N.  North  America 
Northern  Hemisphere 

Asia  Minor,  S.E.  Europe  (?) 
S.  Europe,  E.  Asia 
West  temperate  Asia 
North  America 
N.   Europe,    N.   Africa,    W. 

with  huckleberry. 
Cultivated  for  about  100  years. 
White  and  yellow  varieties  are 
forms. 

California  and  Old  World  grape. 
Many  probably  hybrids. 
Old  and  New  World  species   dis- 

Grapefruit   
Lemon 

B 
B 

Asia,  United  States 
Malayan  and  Pacific  Islands 
east  of  Java 
India 

tinct. 
Largely    cultivated    in      United 

States. 

Orange,  sweet  
Peach  

C 
A 

India 
China 

Numerous  hybrids  with    other 
species. 
Hundreds  of  varieties. 

Pear  

A 

Temperate  Europe  and  Asia, 

Two  species   and    hybrids    be- 

Plum   
Raspberry,  red  

Raspberry,  black  .  .  . 
Strawberry 

A 
C 

F 
F 

N.  China 
S.     Europe,    W.     Asia,     N. 
America. 
N.  Europe,  Asia,  N.  America 

Middle  North  America 
Temperate    N.    America, 

tween  them. 
Much  hybridized    group. 

Varieties  and   hybrids    of     two 
species. 

\t  least  three  species  involved 

Pacific  Coast  of  N.  and  8. 
America,  Europe 

A,  cultivated  for  more  than  4,000  years. 

B,  cultivated  for  more  than  2,000  years. 

C,  cultivated  for  less  than  2,000  years  in  the  Old  World. 

F,  cultivated  since  the  discovery  of  America.     Often  only  very  recently. 

The  mode  of  origin  of  some  of  the  better  United  States  fruit 
varieties  has  been  compiled  by  Dorsey  (1916)  from  the  New  York 
Agricultural  Experiment  Station  fruit  monographs.  A  summary 
statement  is  presented  in  Table  LXVII  I. 

These  data  show  that  nearly  85  per  cent,  of  the  commercial 
fruit  varieties  of  apple,  cherry,  plum,  and  grape  have  been  obtained 
by  selecting  promising  chance  seedlings,  that  one  parent  was 
known  for  a  little  more  than  10  per  cent,  of  the  varieties  described, 


264 


BREEDING  CROP  PLANTS 


TABLE  LXVIII. — ORIGIN   OF  VARIETIES  OF  APPLE,  CHERRY,  PLUM, 
PEACH  AND  GRAPE 


Fruit 

Both 
parents 
known 

One 
parent 
known 

Neither 
parent 
known 

Origin 
as  bud 
sports 

Total 

Apple 

3 

39 

588 

4 

634 

Cherry                

20 

61 

1,064 

n 

1,145 

Grape                                .        ... 

74 

57 

72 

0 

203 

Plum 

49 

108 

524 

1 

682 

Peach 

2 

13 

69 

1 

85 

Total              

148 

278 

2,317 

6 

2,749 

while  over  5  per  cent,  of  the  commercial  varieties  originated  from 
crosses  in  which  both  parents  were  known.  Only  six  out  of 
2,749  varieties  are  known  to  have  originated  as  bud  sports. 

SOME  EARLY  STUDIES  IN  FRUIT  IMPROVEMENT1 

The  preceding  discussion  gives  some  idea  of  the  great  number 
of  varieties  of  our  fruit  crops.  While  many  of  these  are  from 
chance  seedlings,  a  considerable  percentage  resulted  from  definite 
attempts  to  produce  improved  forms. 

Von  Mons. — One  of  the  earliest  horticulturists  was  a  Belgian 
by  the  name  of  Von  Mons,  who  was  born  in  1765  and  died  in 
1842.  He  was  a  chemist  but  followed  horticulture  as  an  avoca- 
tion. His  studies  were  carried  out  for  the  purpose  of  proving  the 
truth  of  certain  philosophical  theories.  While  he  did  not  succeed 
in  substantiating  the  theories,  his  work  was  of  considerable  value 
to  horticultural  science  and  practice.  His  most  important  studies 
were  with  pears.  In  1823  there  were  80,000  seedlings  in  his  nur- 
sery. About  this  time  he  issued  a  catalog  in  which  1,050  pears 
were  described  by  name  or  number.  Of  these,  405  varieties 
were  of  his  own  production.  His  practice  was  to  sow,  select,  and 
resow,  and  without  doubt  a  part  of  his  great  accomplishments 
was  a  direct  result  of  cumulative  selection. 

Knight. — Thomas  Andrew  Knight  has  already  been  mentioned 
as  a  man  who  contributed  much  to  the  art  of  plant  breeding.  He 
was  born  in  England  in  1759  and  died  in  1838.  A  part  of  his  work 
was  carried  on  with  such  fruit  crops  as  apples,  pears,  and  peaches. 

1  For  an  account  of  the  evolution  of  American  fruits  the  reader  is  re- 
ferred to  BAILEY,  1898;  MUNSON,  1906. 


FRUIT  BREEDING  265 

He  emphasized  the  value  of  crossing  as  a  means  of  producing 
improved  forms  for  he  believed  this  method  was  more  rapid  than 
Von  Mons'  selection  practice. 

American  Pomology. — Throughout  the  nineteenth  century 
American  pomologists  made  great  progress  in  the  improvement 
of  fruits.  While  many  American  named  varieties  occurred  as 
chance  seedlings,  others  were  the  result  of  careful  breeding.  The 
strawberry  and  grape  are  examples  of  fruits  in  which  many  of 
the  varieties  are  a  result  of  controlled  breeding.  Selection  and 
crossing  both  played  important  parts  in  the  improvement  of 
varieties.  Hovey  was  one  of  the  best  known  of  the  early  straw- 
berry breeders  who  worked  during  the  first  half  of  the  nine- 
teenth century. 

The  production  of  improved  American  varieties  of  grapes  well 
illustrates  a  common  method  of  the  production  of  new  fruits. 
Old  World  grapes  did  not  succeed  in  the  greater  part  of  the  United 
States,  as  European  varieties  proved  very  susceptible  to  diseases, 
particularly  mildew.  The  production  of  American  varieties  from 
native  wild  species  gave  us  many  of  the  cultivated  types.  Some 
of  the  best  of  the  early  varieties  arose  as  chance  seedlings.  Con- 
cord was  thus  discovered  by  Ephraim  W.  Bull  and  introduced 
about  1853.  It  has  been  frequently  used  as  a  parent  for  the 
production  of  the  improved  forms.  Some  improved  forms  have 
resulted  from  crosses  between  native  and  European  varieties, 
Delaware  being  generally  thought  to  have  been  so  produced. 

With  the  plum,  as  with  the  grape,  the  native  American  species 
have  furnished  the  source  from  which  a  large  part  of  the  American 
varieties  have  been  produced  (Wight,  1915).  Several  wild 
species  have  been  used  and  frequently  the  varieties  which  have 
proved  best  adapted  to  a  given  locality  have  been  produced 
from  the  wild  form  which  is  native  to  the  same  region. 

SOME  CONSIDERATIONS  OF  FRUIT  BREEDING 

Fundamental  laws  of  heredity  furnish  the  same  foundation  for 
a  development  of  correct  breeding  technic  in  the  fruits  as  with 
other  crops.  There  are,  however,  some  factors  which,  modify 
breeding  methods.  For  example,  a  single  tree  takes  up  con- 
siderable field  space  and  thus  has  a  greater  value  than  a  single 
plant  of  wheat  or  corn.  In  comparing  varieties  and  clonal  lines 
the  question  of  soil  heterogeneity  must  be  considered  for  this  is 


266 


BREEDING  CROP  PLANTS 


probably  a  frequent  cause  for  the  variation  in  yield  from  different 
trees  of  the  same  variety  when  grown  in  the  same  orchard.  Self- 
sterility,  which  is  so  prevalent  among  fruit  crops,  often  prevents 
the  production  of  homozygous  material ;  while  the  use  of  heterozy- 
gous material  does  not  allow  the  breeder  to  make  systematic 
crosses  with  a  knowledge  of  the  genetic  constitution  of  the  parents. 
In  spite  of  these  difficulties  which  the  fruit  breeder  must  face, 
there  has  been  a  consistent  attempt  to  use  fundamental  breeding 
principles  and  at  present  methods  are  becoming  somewhat 
standardized.  The  advantage  which  comes  to  the  breeder  from 
the  fact  that  an  improved  variety  may  be  propagated  asexually 
and  need  not  be  reduced  to  a  homozygous  condition,  tends  to 
offset  other  difficulties  Some  of  the  more  general  problems  will 
be  here  illustrated. 

Overcoming  Soil  Heterogeneity. — Batchelor  and  Reed  (1918) 
have  made  an  interesting  study  of  variability  in  orchard  plots. 
They  used  orange,  lemon,  walnut,  and  apple  trees  in  the  investi- 
gation. From  224  to  1,000  trees  of  each  of  the  different  fruits 
were  studied  and  the  coefficient  of  variability  for  yield  of  single 
trees  determined.  The  coefficient  of  variability  of  the  clonal 
varieties  ranged  from  29.72  to  41.23  per  cent.  Thirty-five  per 
cent,  might  be  considered  a  fair  average.  Multiplying  this  by 
0.6745  gives  23.6,  the  probable  error  in  percentage  of  the  mean. 

The  effect  on  the  coefficient  of  variability  of  increasing  the 
number  of  trees  in  a  plot  was  studied ;  a  comparison  of  plots  con- 
taining 1,  2,  4,  8,  16  and  24  trees  being  made.  Table  LXIX  gives 
an  average  of  tests  with  oranges,  lemons,  apples,  and  walnuts. 
The  results  are  based  on  a  study  of  more  than  2,000  individual 
trees. 

TABLE  LXIX. — EFFECT  OF  INCREASING  THE  NUMBER  OF  TREES  PER  PLOT 


Number  | 
of  trees  \ 
per  plot  '; 


Average  coefficient  of 
variability 


Average  reduction  of  coefficient  of  variability  by 
increasing  number  of  adjacent  trees  per  plot 


Increase  from 


Average  reduction 


1 

37.7810.52 

2 

30.89  +  0.55 

1  to    2 

6.89±0.76 

4 

26.76±0.62 

2  to    4 

4.13±0.83 

8 

24.27  +  0.77 

4  to    8 

2.49  +  0.99 

16 

22.  58  ±1.01 

8  to  16 

1.69  +  1.27 

24 

19.74  +  1.08 

16  to  24 

2.84  +  1.48 

FRUIT  BREEDING 


267 


From  these  results  the  conclusion  is  reached  that  eight  trees  is 
about  the  correct  number  which  should  be  used  in  a  plot. 

The  question  of  replication,  i.e.,  the  systematic  distribution  of 
plots  over  the  field,  is  taken  up.  Results  computed  for  four-  and 
eight-tree  units  are  given  for  oranges,  apples,  walnuts,  and 
lemons.  Table  LXX  gives  an  average  of  data  from  these  crops. 

TABLE  LXX. — EFFECT  OF  REPLICATION  IN  FOUR-  AND  EIGHT-PLOT  UNITS 


Four  trees  in  a  unit 

Eight  trees  in  a  unit 

Number  of 
systematically 
replicated  plots 

26.76±0.60 

24.27  +  0.77 

1 

15.12±0.47 

12.84±0.56 

2 

13.58±0.53 

11.27±0.63 

3 

9.29  +  0.40 

9.54±0.57 

4 

8.  40  ±0.40 
8  49  +  0  49 

7.95  +  0.49 

5 

6 

The  conclusion  seems  warranted  that  four  systematically 
replicated  plots  greatly  reduces  the  error  which  arises  from  soil 
heterogeneity.  The  data  also  show  that  four  systematically 
distributed  plots  of  four  trees  each  are  somewhat  more  reliable 
than  two  plots  of  eight  trees  each. 

As  was  presented  in  Chapter  IV,  Harris  has  given  a  reliable 
means  of  estimating  soil  heterogeneity  by  the  correlation  between 
the  neighboring  plots  of  a  field.  The  test  was  applied  to  an 
orange  grove  which'  appeared  to  have  uniform  soil  conditions. 
The  correlation  between  the  yield  of  eight-tree  plots  as  ultimate 
units  and  grouped  combinations  of  four  such  adjacent  plots 
was  found  to  be:  r=  +0.533  +  0.085.  This  showed  a  pro- 
nounced heterogeneity  in  the  soil  of  this  orchard.  However,  the 
correlation  computed  between  the  yield  of  an  eight-tree  ultimate 
unit  and  the  yield  of  the  combination  of  four  such  systematically 
distributed  units  was  not  much  larger  than  the  probable  error. 

These  facts  show  the  unreliability  of  yields  of  single  trees  as  a 
criterion  of  productivity,  that  eight-tree  plots  give  much  more 
reliable  results,  and  that  plot  replication  is  of  as  much  value  in 
studies  of  fruit-yield  as  of  farm  crops.  Where  quality  is  a  major 
criterion,  single  trees  give  fairly  reliable  information. 

Self -sterility  and  Heterozygosity. — One  of  the  chief  difficul- 
ties of  systematizing  methods  of  work  is  due  to  the  heterozyogus 


268 


BREEDING  CROP  PLANTS 


condition  of  most  fruit  material.  A  commercial  variety  may  be 
extremely  valuable  and  yet  be  heterozygous  for  many  characters. 
On  the  other  hand,  the  commercial  variety  may  be  homozygous 
for  a  large  part  of  its  characters.  It  seems  reasonable  to  con- 
clude that  the  more  nearly  homozygous  the  parental  variety, 
other  things  being  equal,  the  greater  value  it  would  have  as  a 
parent. 

The  ability  of  impressing  its  characteristics  upon  the  larger 
part  of  its  offspring  has  been  called  prepotency  by  animal  breeders. 
Such  prepotency  is  genetically  explained  by  the  supposition  that 
the  prepotent  parent  is  homozygous  for  certain  dominant  factors 
for  the  characters  under  observation.  Hedrick  and  Wellington 
(1912)  showed  that  some  crosses  between  apple  varieties  pro- 
duced a  considerable  percentage  of  individuals  with  small  fruits. 
Thus  the  cross  between  Rails  and  Northern  Spy  gave  great 
variability  in  size  of  apples,  while  the  cross  between  Sutton  and 
Northern  Spy  gave  progeny  in  which  no  trees  were  obtained 
which  produced  small  fruit.  One  of  the  great  difficulties  is  that 
it  takes  several  years  to  learn  the  varieties  which  when  crossed 
will  give  certain  desired  combination. 

Another  difficulty  which  must  be  considered  is  that  many 
varieties  of  fruits  are  self-sterile.  This  is  of  utmost  importance 
in  commercial  fruit  production  for  it  is  necessary  to  interplant 
such  a  variety  with  some  variety  which  produces  an  abundance  of 
pollen  which  is  capable  of  fertilizing  the  variety  in  question  and 

TABLE  LXXI. — INCREASE  IN  WEIGHT  OF  SEED  AND  FRUIT  DUE  TO  CROSS- 
POLLINATION 


Pollination 

Average 
weight  of 
seed,  grams. 

Average 
weight  of 
fruit,  grams. 

Newtown  X  self    

0  05 

73 

Newtown  X  Bellflower 

0  40 

104 

Newtown  X  Spitzenberg  

0  66 

147 

Newtown  X  Jonathan  

0  65 

162 

Newtown  X  Grimes  Golden 

0  60 

173 

Spitzenberg  X  self  

0  13 

100 

Spitzenberg  X  Newtown  

0  65 

126 

Spitzenberg  X  Arkansas  Blk 

0  68 

128 

Spitzenberg  X     Jonathan 

0  70 

144 

Spitzenberg  X  Baldwin  

0.71 

157 

FRUIT  BREEDING 


269 


which  blooms  at  about  the  same  period.  The  self-sterile  habit 
likewise  prohibits  the  reduction  of  the  material  to  a  homozygous 
condition.  Frequently  self-fertile  varieties  give  great  increases 
in  weight  of  seed  and  fruits  as  a  result  of  cross-pollination.  There- 
fore, pollinators,  varieties  which  have  proved  desirable  as  pollen 
parents,  are  often  of  considerable  commercial  value  in  increasing 
yield  in  the  case  of  self-fertile  varieties. 

Table  LXXI  gives  two  typical  cases  taken  from  the  work  of 
Lewis  and  Vincent  (1909)  with  the  apple. 

The  large  increases  in  weight  of  seed  as  a  result, of  crossing  are 

TABLE  LXXII. — CONDENSED  STATEMENT  OF  NUMBER  OF  SELF-FERTILE, 
SELF-STERILE,  AND  PARTIALLY  SELF-FERTILE  VARIETIES  OF  FRUIT  CROPS 


Number 

Number 

Number 

| 

Fruit 

self- 

self- 

partially 

Authority                           Remarks 

fertile 

sterile 

self-sterile 

Grape  7 

13 

5 

Dorsey,  1914 

Self-fertile  and  partially  self- 

I 

after    Beach 

fertile  have  upright  stamens 

(1898,  1899) 

and  pollen  with  germ  pore. 

Self-sterile  varieties  have  re- 

flexed    stamens    and    pollen 

with  no  germ  pore. 

Grape  |        .  .-"• 

Beach,  1902         Pollen  of  self-sterile  varieties 

I 

can  not  fertilize  other  self- 

sterile  varieties. 

i 

Plum  

All     cultivated      varieties     of 

Dorsey,  1919 

Results  given  by  Dorsey   are 

native   American   species   ex- 

from    studies     of     Waugh 

cept  New  Ulm  and  Robinson 

(1896,     1897,     1898,     1899, 

are  self-sterile. 

1900,      1901)      Goff      (1894, 

1901),  and  Waite  (1905). 

Plum  

18 

16 

5 

Sutton,  1918 

All    self-sterile    varieties     set 

fruit   when  pollinated     with 

any  other  variety  with  few 

exceptions. 

Cherries  .  .  . 

3 

17 

2 

Sutton,  1918 

Apples  .... 

8 

16 

10 

Sutton,  1918 

Apples  ... 

28 

59 

Lewis  and  Vin- 

From 50  to  200  pollinations 

cent,  1909 

were  made  for  each  variety. 

If  no  seed  set,  variety  is 

« 

classed    as    self  -sterile.     All 

varieties     with     some    seed 

setting    are    classed  as  self- 

fertile,     although    some    are 

partially  self-sterile. 

Pears  j  Bartlett  and  Kieffer  pears  are 

Fletcher,  1911 

self-sterile. 

270  BREEDING  CROP  PLANT* 

very  noticeable.  Increases  in  size  of  fruit  are  also  of  much 
importance. 

For  the  commercial  grower  or  the  fruit  breeder,  it  is  essential  to 
know  which  varieties  are  self-sterile.  In  order  to  illustrate  the 
conditions  generally  found  regarding  sterility,  a  compilation  of 
some  results  is  presented  in  Table  LXXII.  Citations  to  literature 
are  given  so  that  the  reader  may  go  to  the  original  sources  when 
he  desires  to  know  what  category  any  particular  variety  belongs 
to. 

The  causes  of  sterility  have  been  determined  in  some  cases. 
In  the  strawberry  it  is  due  to  at  least  two  causes  (Valleau,  1918) : 

1.  The  dioecious  condition. 

2.  The  production  of  aborted  pollen  grains  or  microspores  in 
otherwise  normal  anthers. 

In  the  grape,  Dorsey  (1914)  has  found  sterility  to  be  asso- 
ciated with  both  hybridity  and  the  dioecious  condition.  The 
varieties  which  produce  reflexed  stamens  seldom  produce  fertile 
pollen.  Dorsey  states  that : 

"  Sterility  has  been  found  to  be  due  to  the  pollen  rather  than  in  the 
pistil.  Sterile  pollen  in  the  grape  results  from  degeneration  processes 
in  the  generative  nucleus  or  arrested  development  previous  to  mitosis 
in  the  microspore  nucleus." 

Pollen  abortion  occurs  both  in  pure  and  hybrid  forms  but  is  not 
considered  a  cause  of  lack  of  fertility  as  abundant  pollen  is  pro- 
duced in  the  grape. 

In  the  plum,  pollen  abortion  is  not  as  a  rule  the  cause  of  self- 
sterility.  The  outstanding  features  as  given  by  Dorsey  (1919) 
are: 

"(a)  A  constancy  of  expression  of  self-sterility  even  in  P.  domestica 
in  which  about  one-half  of  the  varieties  are  self-f ertile ;  (b)  the  occur- 
rence of  cross-sterility;  and  (c)  the  slow  growth  of  pollen  tubes  under 
the  condition  of  self-  and  cross-sterility." 

This  type  of  sterility  is  comparable  with  that  in  the  tobacco 
crosses  previously  discussed,  where  sterility  resulted  from  slow 
pollen  tube  growth.  In  this  case  the  pollen  tube  growing 
from  the  pollen  grain  into  the  tissues  of  the  style  never  reaches  the 
embryo  sac.  The  self-sterile  condition  is  believed  by  Dorsey  to 
be  a  dominant  character  in  the  plum  and  to  be  inherited,  segrega- 
tion into  sterile  and  fertile  forms  occurring  at  reduction  division. 


FRUIT  BREEDING  271 

Knight  (1917)  has  made  a  study  of  self-sterility  in  the  apple  and 
the  conclusions  reached  show  the  manifold  causes  which  must  be 
considered  in  a  study  of  the  problem.  For  this  reason  the 
conclusions  are  here  given  verbatim. 

"1.  Self -sterility  in  Rome  Beauty  is  not  due  to  sterility  of  the  pollen 
as  has  been  shown  to  be  the  case  in  certain  varieties  of  grapes. 

"2.  Sensitiveness  of  pollen  to  over-abundant  moisture  supply  is  not 
involved  here  as  a  factor,  as  has  been  shown  by  Jost  for  the  pollen  of 
many  grasses,  barley  especially;  and  by  J.  N.  Martin  for  the  pollen  of 
red  clover.  The  pollen  of  Rome  Beauty  and  many  other  varieties 
germinated  in  distilled  water. 

"3.  Rome  Beauty  stigmatic  fluid  extracts  offer  no  inhibition  to  the 
germination  and  growth  of  Rome  Beauty  pollen. 

"4.  Rome  Beauty  stigmas  offer  no  particular  mechanical  obstruction 
to  the  penetration  of  Rome  Beauty  pollen  tubes. 

"5.  Self -sterility  of  Rome  Beauty  is  not  due  to  inability  of  its  own 
pollen  tubes  to  grow  deep  enough  to  reach  the  egg.  This  has  been  sug- 
gested as  the  cause  of  self-sterility  in  certain  pear  and  apple  varieties 
by  the  work  of  Osterwalder. 

"6.  From  present  indications  one  important  factor  in  self -sterility 
of  Rome  Beauty  is  the  relatively  slow  rate  of  growth  of  Rome  Beauty 
tubes  in  Rome  Beauty  stylar  tissue.  Doubtless  other  factors  will  be 
found  upon  further  examination." 

Inheritance  of  Some  Characters. — The  mode  of  inheritance 
of  most  fruit  characters  has  as  yet  not  been  determined.  There 
are,  however,  numerous  experiments  under  way  for  the  purpose  of 
learning  how  individual  characters  behave  in  crosses.  The  lack 
of  information  in  this  field  is  due  to  the  heterozygous  condition 
of  many  fruit  varieties  and  to  the  fact  that  with  many  fruit  crops 
so  long  a  period  elapses  between  the  time  of  sowing  the  seed  and 
the  production  of  fruit. 

Apple. — Inheritance  in  the  apple  is  well  illustrated  by  a 
study  made  at  the  Geneva  Station  by  Hedrick  and  Wellington 
(1912).  Crosses  were  made  in  1898  and  1899  and  148  seedlings 
were  grown.  In  1912,  106  of  the  seedlings  had  come  into  bearing. 
These  106  seedlings  resulted  from  11  crosses.  The  first  genera- 
tion naturally  does  not  furnish  very  reliable  data  as  a  means  of 
deciding  the  mode  of  inheritance  of  individual  characters. 

Three  types  of  skin  color  were  studied,  red,  yellow,  and  inter- 
mediates. The  conclusion  was  reached  that  Ben  Davis  and 
Jonathan  were  both  pure  for  red  color  of  skin,  as  crosses  between 


272 


BREEDING  CROP  PLANTS 


these  varieties  gave  seedlings  which  produced  fruit  with  a  red 
skin.  Other  crosses  led  to  the  belief  that  yellow  is  recessive 
and  that  a  cross  between  red  and  yellow  is  intermediate  in  skin 
color.  Sweetness  was  believed  to  be  a  recessive  character  to 
acidity  with  the  indication  that  the  F\  was  intermediate. 

Raspberry. — Bailey  (1898)  believed  that  the  purple  rasp- 
berry, Rubus  neglectus,  was  a  natural  hybrid  between  the  black 
and  red  varieties.  This  was  definitely  proved  at  the  Geneva 
Station  by  a  cross  between  Smith  No.  1,  a  black  raspberry, 
and  Lonboro,  a  red  seedling,  which  gave  209  purple  raspberries 
(Wellington,  1913,  Anthony  and  Hedrick,  1916).  The  same  Smith 
No.  1  crossed  with  June,  a  red  raspberry,  gave  50  purples  and 
46  blacks.  Selfed  seedlings  of  Columbian,  a  purple  variety,  gave 
31  purple,  7  red  wine,  2  reddish,  1  yellow,  and  1  black.  The  mode 
of  inheritance  of  colors  can  not  be  determined,  although  it 
seems  that  several  of  the  black  varieties  are  heterozygous  for 
color  and  that  several  factors  for  color  are  present.  The  presence 
of  bloom  on  the  canes  proved  to  be  a  partially  dominant  charac- 
ter over  the  absence  of  bloom.  The  number  of  spines  on 
canes  showed  segregation  in  selfed  seedlings  of  Columbian. 
Yellow  raspberries  could  be  told  in  the  seedling  stage  from  the 
black  and  purple  by  the  absence  of  red  tinge  on  the  leaves.  The 
production  of  promising  varieties  from  crosses  between  the  red 
and  black  varieties  was  especially  mentioned. 

Grape. — The  Geneva  Experiment  Station,  in  New  York, 
(Hedrick  and  Anthony  1915)  likewise  furnished  the  greater  part 
of  our  data  on  inheritance  of  characters  in  the  grape.  Table 
LXXIII  gives  the  results  of  crosses  for  skin  color. 

TABLE  LXXIII. — INHERITANCE  OF  SKIN  COLOR  IN  GRAPES 


Color  of  parental  types 

Color  of  seedlings 

Black 

Purple 
to  dark 
red 

Medium    ! 
to  light          White 
red 

White  X  white  

6 
43 
49 
44 
3 
52 

13 

45 
13 

1 

40 

166 

8 
42 
54 
50 
12 
32 

Light  red  X  light  red 

8 
38 
407 
5 
41 
100 

Dark  red  X  dark  red  

Black  X  black  
White  X  dark  red  
White  X  black                 .... 

Black  X  dark  red 

FRUIT  BREEDING  273 

The  chief  conclusions  which  may  be  reached  from  these  results 
are  that  nearly  all  varieties  are  heterozygous  for  color  and  that 
white  is  a  pure  recessive. 

In  studies  of  inheritance  of  quality  there  is  a  proof  of  the  value 
of  selecting  as  parents  the  types  which  excel  for  the  character 
being  worked  with.  Table  LXXIV  gives  some  of  the  results 
of  crosses  in  which  quality  was  studied. 

TABLE  LXXIV. — INHERITANCE  OF  QUALITY  IN  THE  GRAPE 


Parental  types 

Total 

Percentage 
of  good  or 
better 

Parents  good  or  of  higher  quality 

682 

27 

Good  X  fair  or  poor        

56 

11 

Medium  X  medium 

213 
" 

10 

Poor  X  poor 

51 

4 

Nearly  all  grapes  of  high  quality  at  the  New  York  station 
contain  some  V.  vinifera  blood.  This  is  easily  understood 
when  one  remembers  the  long  period  of  breeding  of  the  European 
varieties  and  that  American  varieties  were  only  recently  obtained 
from  the  wild.  Inheritance  of  size  of  grape  berry  and  ripening 
period  showed  the  value  of  selecting  as  parents  varieties  which 
excel  in  the  character  which  the  breeder  wishes  to  obtain. 

Illustrations  of  Methods  of  Breeding. — Methods  of  breeding 
fall  naturally  under  three  main  heads; 

1.  Selection  of  bud  sports. 

2.  Seedling  selection. 

3.  Controlled  crosses. 

As  has  been  already  mentioned  many  of  our  varieties  have 
resulted  from  chance  seedlings,  others  from  seedlings  in  which 
only  one  parent  was  known.  A  review  of  the  subject  leads  to  the 
conclusion  that  the  improvement  of  fruits  by  the  use  of  self- 
fertilized  seed  is  a  less  desirable  method  than  by  the  use  of  crossed 
seed.  When  selfed  seed  can  be  produced  the  progeny  are  as  a 
rule  less  vigorous  than  those  obtained  from  crossed  seed.  As 
these  subjects  have  been  touched  upon  in  some  detail  under  other 
headings,  seedling  selection  will  not  be  discussed  further. 

Selection  of  Bud  Sports. — It  is  now  a  commonly  accepted  fact 
that  mutations  or  sudden  changes  in  the  germinal  material  do 
occasionally  occur.  Likewise,  in  asexually  propagated  species 

18 


274  BREEDING  CROP  PLANTS 

bud  sports  have  been  found,  and  in  some  cases  these  have  been 
used  as  the  foundation  of  improved  races.  To  justify  a  method 
of  breeding  founded  upon  their  utilization,  such  bud  sports 
must  occur  frequently  enough  to  pay  for  the  trouble  of  making 
a  systematic  search  for  them. 

A  review  of  the  experimental  evidence  is  of  considerable 
interest,  for  this  is  the  only  means  we  have  of  deciding  whether 
the  selection  of  particular  trees  or  branches  for  propagating  pur- 
poses is  a  reliable  means  of  producing  new  varieties.  Of  the  four 
apple  bud  sports  mentioned  in  Table  LXVIII  the  chief  changes 
were  in  the  color  of  the  fruits.  In  the  Isabella  grape  several 
sports  were  obtained  which  produced  black  grapes  of  larger  size 
than  Isabella,  and  which  excelled  in  sweetness  (Powell,  1898  cited 
from  Dorsey,  1916).  Dorsey  (1916)  records  two  large-fruited 
variations  in  the  Concord  grape  which  arose  as  bud  sports. 

Instances  of  bud  variations  in  ornamental  horticultural  plants 
are  quite  common.  As  an  example  of  their  frequency,  the  work 
of  Stout  (1915)  will  be  briefly  discussed.  Extensive  asexual  or 
clonal  selections  were  made  in  Coleus  and  numerous  color  changes 
were  isolated  as  well  as  changes  in  leaf  shape.  The  same  varia- 
tions were  obtained  through  bud  sports  as  by  seed  reproduction. 
Some  clonal  lines  sported  much  less  frequently  than  others. 

The  work  on  citrus  fruits  (Shamel  and  others,  1918)  which  has 
been  carried  on  in  California,  has  drawn  the  attention  of  many 
horticulturists  and  plant  breeders  to  the  subject  of  bud  sports  and 
their  place  in  correct  fruit-breeding  methods.  Valencia  oranges 
were  originally  introduced  from  three  sources,  but  all  have  proved 
of  similar  type  and  are  now  called  Valencia.  From  this  variety 
12  important  strains  originating  as  bud  sports  have  been  isolated. 
As  a  rule,  single  off-type  branches  produce  fruits  showing  charac- 
ters which  are  different  from  the  fruits  borne  on  the  remainder 
of  the  tree.  Many  of  these  sports  are  of  highly  undesirable  type. 
The  Washington  navel  orange  was  introduced  from  Brazil  in 
1870  by  the  Department  of  Agriculture  at  Washington.  Thir- 
teen distinct  strains  have  been  isolated  through  bud  selection. 
Thompson,  one  of  these  strains,  has  proved  a  very  desirable  type. 
Likewise,  bud  sports  have  occurred  in  the  grapefruit  which 
was  introduced  in  California  from  Florida  in  1890.  The  Marsh 
is  the  best  of  six  strains  which  were  obtained  by  selecting  bud 
sports.  Similarly  bud  sports  have  occurred  in  lemon  orchards. 
Shamel  (1919)  records  an  occurrence  of  a  sporting  branch  in  a 


FRUIT  BREEDING  275 

French  prune  tree  which  was  first  observed  in  1904.  Several 
grafts  from  this  branch  were  placed  in  bearing  trees.  These 
grafts  reproduced  the  characters  of  the  sporting  branch.  In 
1914,  trees  in  alternate  rows  of  an  orchard  were  top-worked  by 
the  use  of  buds  from  the  new  strain  and  compared  with  buds  from 
the  normal  French  prune  variety.  The  top-worked  trees  from  the 
bud  sport  bore  larger  fruit  than  those  from  the  normal  prune. 
The  fruits  were  also  more  evenly  distributed  over  the  tree  than 
in  the  original  French  prune  variety. 

The  above  are  some  of  the  more  striking  instances  of  the  pro- 
duction of  new  varieties  through  the  isolation  of  bud  sports. 
Crandall  (1918)  has  made  an  extensive  test  in  Illinois  of  the  value 
of  bud  selection  in  apples  as  a  means  of  improving  the  variety. 
Two  distinct  lines  of  study  have  been  followed. 

1.  The  value  for  propagating  purposes  of  buds  selected  in 
different  ways.     The  experiments  included  a  comparison  of  large 
versus  small  buds,  of  buds  from  different  parts  of  the  tree  and 
from  different  locations  on  the  shoot. 

2.  Selection  of  trees  because  of  special  merit.     Comparison 
of  seedlings  produced  from  large  and  small  apples  produced  by 
these  selected  trees. 

A  considerable  number  of  varieties  was  used  for  the  first 
study  and  a  total  of  5,400  buds  were  selected.  A  careful  measure- 
ment was  then  made  of  the  yearly  growth  of  wood  from  the  buds 
which  had  been  previously  selected.  Growth  curves  were  made 
and  on  the  basis  of  these  results  the  conclusion  was  reached  that 
all  buds  from  healthy  shoots  were  of  equal  value  for  propagation 
purposes. 

The  characters  of  seedlings  grown  from  seeds  of  large  and 
small  fruits  borne  on  trees  of  special  merit  were  carefully  studied. 
Seeds  from  large  fruits  produced  seedlings  which  were  somewhat 
more  resistant  to  adverse  conditions  than  seedlings  grown  from 
small  fruits.  The  hypothesis  that  this  may  be  explained  by  the 
fact  that  large  fruits  and  large  seeds  frequently  occur  from  crosses, 
seems  reasonable  in  the  light  of  the  work  of  Lewis  and  Vincent 
previously  cited. 

Stewart  (1912)  has  discussed  the  value  of  cion  selection  in  tree- 
fruit  improvement.  Individual  apple  tree  data  over  a  period  of 
from  ten  to  fourteen  years  were  presented.  Under  apparently  the 
same  conditions  some  trees  were  consistently  higher  yielders 
than  others.  A  review  of  considerable  experimental  evidence  led 


276  BREEDING  CROP  PLANT* 

Stewart  to  conclude  that  there  was  more  evidence  in  favor  of 
purity  of  the  clone  than  in  favor  of  the  value  of  clonal  selection 
as  a  means  of  producing  higher-yielding  strains.  Similar  con- 
clusions were  reached  from  an  experiment  carried  on  by  Tyson 
brothers,  in  New  York,  with  the  York  Imperial  apple.  Two 
trees  were  selected  which  bore  unusually  similar  fruits  and  these 
were  used  for  propagation.  More  than  8,000  trees  were  planted 
in  the  new  orchard.  Examination  of  trees  of  this  orchard  when 
they  came  into  bearing  showed  them  to  be  not  superior  to  the 
usual  York  Imperial  apple  (Dorsey,  1917). 

The  cited  cases  show  the  present  status  of  the  problem  of  selec- 
tion of  bud  sports  as  a  means  of  improvement  of  fruit  crops. 
The  studies  with  the  citrus  genus  appear  to  justify  the  belief 
that  degenerate  or  inferior  bud  sports  are  of  frequent  occurrence. 
This  leads  to  a  conclusion  that  only  those  limbs  which  produce 
normally  healthy  fruit  should  be  used  for  propagation  purposes. 
Even  among  the  citrus  fruits  there  is  as  yet  no  very  conclusive 
proof  that  the  selection  of  cions  from  high-yielding  trees  will 
accomplish  more  than  to  prevent  possible  "running  out"  of  the 
variety.  The  evidence  from  apples  would  seem  to  justify  the 
belief  that  bud  sports  are  very  infrequent.  The  breeder,  then, 
can  well  afford  to  make  careful  observations  with  the  hope  of 
discovering  bud  sports.  If  apparently  desirable  sports  are  found, 
these  may  then  be  used  for  propagation. 

In  such  crops  as  citrus  fruits  and  with  such  plants  as  Coleus, 
bud  sports  are  of  frequent  occurrence.  There  is,  then,  some  evi- 
dence for  the  belief  that  sports  occur  more  frequently  in  hetero- 
zygous than  in  homozygous  material.  As  Stout  (1915)  obtained 
the  same  changes  through  asexual  selection  as  by  the  use  of  self- 
fertilized  seed,  it  seems  reasonable  to  suppose  that  some  sort  of 
segregation  and  recombination  occurs  in  somatic  tissue.  No 
cytological  evidence  has  been  given  to  account  for  such  a  supposi- 
tion. With  heterozygous  material  the  loss  of  a  single  dominant 
factor  would  be  immediately  apparent  in  the  soma.  This  is  one 
reason  why  bud  sports  occur  more  frequently  in  heterozygous 
forms  (East  and  Jones,  1919).  Nabours  (1919)  has  shown  that 
similar  cross-overs  occur  in  parthenogenetic  reproduction  in  the 
grouse  locust  as  in  those  forms  which  are  produced  by  the  recom- 
bination of  gametes  containing  the  haploid  number  of  chromo- 
somes. If  the  usual  sort  of  cross-overs  occurred  in  homozygous 
material,  there  would  be  no  change  in  the  homologous  parts  of 


FRUIT  BREEDING  277 

chromosome  pairs.  In  heterozygous  material,  however,  new 
combinations  of  factors  would  be  produced  which  might  cause 
changes  in  the  external  appearance  of  the  organism.  No  cyto- 
logical  basis  for  such  cross-overs  has  been  demonstrated. 

Controlled  Crosses. — One  of  the  earliest  controlled  experi- 
ments in  the  breeding  of  fruits  by  crossing  was  started  by  Swingle, 
in  1893,  in  Florida.  This  was  an  attempt  to  produce  hardier 
types  by  the  use  of  wild  citrus  species.  The  hardy  Chinese  species, 
Citrus  trifoliata,  was  used  as  one  of  the  parents.  In  1897,  212 
crosses  were  made  between  this  species  and  orange  varieties. 
The  three  fruits  that  were  produced  gave  thirteen  hybrids,  which 
were  so  different  from  existing  varieties  of  citrous  fruits  that 
they  were  called  "Citranges."  Other  crosses  between  citrous 
species  were  made.  One  of  the  promising  combinations  was  a 
cross  between  the  West  India  lime  and  the  kumquat  orange. 
This  orange  is  one  of  the  hardiest  of  the  evergreen  citrous  trees 
while  the  lime  is  very  tender.  Further  experiments  are  under 
way  and  other  promising  wild  relatives  of  the  citrous  fruits  have 
been  obtained.  Crosses  of  this  nature  are  producing  fruit  varie- 
ties which  are  successful  in  regions  where  citrous  fruits  could 
not  be  grown  formerly.  The  work  shows  the  necessity  of  a 
thorough  botanical  knowledge  of  the  wild  relatives  of  the  crop 
which  it  is  hoped  to  improve  by  breeding. 

A  somewhat  similar  method  of  work  with  the  hope  of  producing 
hardy  apples  for  the  Canadian  Northwest  was  started  by  William 
Saunders  in  Canada  in  1888.  The  wild  Siberian  crab,  Pyrus 
baccata,  which  proved  hardy  on  the  prairies  and  withstood 
temperatures  of  50°  below  zero,  was  used  as  the  female  parent  and 
crossed  with  commercial  apple  varieties.  Macoun  (1915)  states 
that  the  fruit  of  Pyrus  baccata  averages  %  in-  in  diameter  and  is 
quite  astringent.  The  fruits  obtained  from  some  of  the  more 
promising  ol  the  crosses  were  not  so  large  as  desired,  although 
some  compared  very  favorably  in  size  with  ordinary  crabs. 
They  were  of  good  flavor  and  proved  hardier  than  any  varieties 
of  apples  and  crabs  that  had  been  tested  up  to  that  time.  Several 
are  here  listed. 

Jewel,         P.  baccata  X  Yellow  Transparent.     Size  1.4  by  1.3  in. 
Columbia,  P.  baccata  X  Broad  Green.  Size  1.8  by  1.6  in. 

Charles,      P.  baccata  X  Tetofsky.  Size  1.6  by  1.5  in. 

Recrosses  between  the  best  of  these  and  apple  varieties  were 


278  BREEDING  CROP  PLANTS 

made  and  407  trees  were  grown.  Some  varieties  were  obtained 
with  larger  fruits  but  these  as  yet  have  not  been  thoroughly 
tested  for  hardiness. 

Pears  have  been  frequently  tried  in  the  Dakotas  but  have 
failed  for  two  causes  (Hansen,  1915):  (1)  Lack  of  hardiness; 
(2)  susceptibility  to  blight.  The  Chinese  sandpear,  Pyrus 


FIG.  64. — Wolf,  a  hardy  variety  of  plums  which  lacks  quality  of  fruit.     (Photo 

loaned  by  Dorsey.) 

sinensis  Lindley,  obtained  from  Dr.  Sargent,  of  the  Arnold 
Arboretum,  proved  perfectly  hardy  and  resistant  to  blight. 
Various  crosses  between  this  species  and  cultivated  pears  be- 
longing to  Pyrus  communis  have  been  made.  Preliminary  tests 
have  shown  that  some  of  the  seedlings  were  blight  resistant  and 
hardy.  These  results  indicate  that  the  problem  of  producing 


FIG.  65. — Burbank,  a  plum  of  high  quality  produced  by  Luther  Burbank.     It 
lacks    hardiness   when    grown   in    Minnesota.     (Photo  loaned  by  Dorsey.) 

pears  for  the  Northwest  may  eventually  be  solved.  In  a  some- 
what analogous  manner,  Hansen  (1911)  has  produced  new  plum 
varieties  by  crossing  the  native  sand  cherry  with  Japanese  plums. 
This  has  resulted  in  a  "happy  combination  of  hardiness,  rapid 
growth  and  early  bearing  of  tree,  with  large  size  and  choice 
quality  of  fruit." 


FRUIT  BREEDING 


279 


It  will  be  of  interest  here  to  present  briefly  an  instance  from 
the  fruit-breeding  work  at  the  Minnesota  Station  in  which 
desirable  new  plum  hybrids  were  obtained  when  the  tender 
parent,  Burbank  (P.  triflora)  was  crossed  with  Wolf  which  is  a 
hardy  variety  of  P.  americana  mollis.  The  percentage  of  hy- 
brids killed  during  winter  dormancy  is  taken  as  a  basis  for  clas- 
sification. It  will  ,be  seen  that  some  of  these  hybrids,  as  No.  8 
or  No.  9,  are  hardy  in  the  bud  like  the  staminate  parent  Wolf. 
The  two  which  have  been  named  Red  Wing  and  Tonka,  are  inter- 
mediate in  hardiness  but  of  excellent  fruit  characteristics. 


FIG.  66. — Tonka,  Burbank  X  Wolf,  No.  21.     Has  high  quality  and   is  nearly 
as  hardy  as  the  hardy  variety  of  Wolf.     (Photo  loaned  by  Dorsey.} 

TABLE  LXXV. — SHOWING  THE   PERCENTAGE  OF  BUDS  KILLED  IN  AN  FI 
PROGENY  WHEN  ONE  OF  THE  PARENTS  is  HARDY  AND  THE  OTHER  TENDER1 


Parent 

Percentage  of 
buds  killed 
1916-17 

Parent 

Percentage  of 
buds  killed 
1917-18 

Burbank 

100 

Hybrid  No.    9  

0 

Wolf  

0 

10  

50 

Hybrid  No   1 

50 

11 

1 

2 

35 

12  (Red  Wing) 

10 

3 

5 

14                .    . 

25 

4  

5 

15  

5 

5 

10 

16  

5 

6 

10 

17                ... 

0 

7 

0 

20 

25 

8       .    .. 

0 

21  (Tonka)  

25 

These  few  instances  have  been  given  as  indicative  of  the 
methods  of  work  which  are  being  used  by  some  of  the  most  pro- 
gressive fruit  breeders.  Some  general  conclusions  regarding 
methods  of  work  may  be  here  given. 

1  Data  furnished  by  M.  J.  Dorsey. 


280  BREEDING  CROP  PLANTS 

1.  A  knowledge  of  the  botanical  relationship  and  wild  relatives 
of  the  fruit  are  necessary  if  greatest  progress  in  improvement  is 
to  be  obtained. 

2.  Some  varieties  and  species  transmit  their  characters  to  a 
much  greater  degree  than  do  other  varieties.     A  knowledge  of 
the  more  prepotent  varieties  materially  aids  in  planning  a  cross. 

3.  Varieties  selected  as  parents  should  contain  in  the  highest 
degree  possible  the  character  or  characters  desired  in  the  progeny. 

4.  The  larger  the  numbers  of  progeny  grown,  the  greater  the 
chances  of  obtaining  the  combination  desired. 

5.  Most  fruit  crosses  give  variable  progeny  in  FI.     Numerous 
crosses  should,  therefore,  be  made. 

6.  Information  regarding  the  mode  of  inheritance  of  particular 
characters  will  assist  in  selection  of  varieties  to  be  used  as  parents. 


CHAPTER  XIX 
FARMERS'  METHODS  OF  PRODUCING  PURE  SEEDS 

The  production  of  new  varieties  of  farm  crops  is  a  specialized 
line  of  work  and  should  be  undertaken  as  a  rule  only  by  men 
who  have  had  special  training  in  crop  breeding.  •  The  expense 
and  time  necessary  for  this  kind  of  experimental  work  are  too 
great  for  the  individual  farmer.  The  aim  of  the  farmer  or  seed 
grower  should  be  to  maintain  the  improved  form  and  not  to  allow 
contamination  through  crossing  with  inferior  stock,  admixture, 
plant  diseases,  etc.  A  method  of  producing  seed  which  will 
stand  this  test  and  at  the  same  time  meet  with  the  approval  of 
the  farmer  must  be  simple,  effective,  and  inexpensive.  A  nation 
or  state  cannot  afford  to  maintain  an  experimental  laboratory 
only  to  have  the  products  of  that  laboratory  deteriorate  because 
of  subsequent  treatment.  The  maintenance  of  pure,  improved 
varieties  as  well  as  their  discovery  by  selection  or  synthesis  by 
crossing  is  an  essential  factor  in  economic  food  production.  Be- 
fore taking  up  in  detail  methods  of  producing  pedigreed  seed  by 
farmers,  a  few  observations  regarding  seeds  in  general  will  be 
made. 

DETERMINATION  OF  BETTER  VARIETIES 

Certain  general  facts  regarding  varieties  should  be  understood. 
The  breeder  and  grower  must  recognize  that  no  one  variety  is  best 
adapted  to  a  particular  locality  or  for  all  seasons.  In  some  seasons 
an  early  oat  gives  the  best  yield.  Owing  to  slight  seasonal 
variations,  a  later  variety  may  excel  in  yield.  Thus,  a  single 
season's  test  is  not  reliable  as  a  means  of  determining  the  better 
sort  to  grow.  For  this  reason  carefully  conducted  tests  are 
carried  on  each  crop  season.  By  means  of  these  the  experiment 
stations  are  in  a  position  to  determine  and  advise  as  to  the 
better  varieties.  The  final  decision  as  to  which  variety  to  grow 
must  of  course  be  made  by  the  farmers  and  based  on  their  act- 
ual field  experiences. 

281 


282  BREEDING  CROP  PLANTS 

WHAT  IS  GOOD  SEED? 

There  are  certain  characters  of  farm  crops  which  must  be 
considered  if  the  grower  wishes  to  produce  good  seed.  Good 
seed  of  any  farm  crop  must  belong  to  a  variety  that  is  superior 
in  the  following  respects : 

1.  Adaptability  to  the  locality  and  soil. 

2.  Yielding  ability. 

3.  Purity  to  type  for  small  grains  or  self-pollinated  crops,  and  comparative 
purity  for  corn  and  other  cross-pollinated  crops. 

4.  Quality  for  the  particular  characters  for  which  the  crop  is  grown. 

5.  Hardiness. 

6.  Erectness  or  ability  to  withstand  lodging. 

7.  Disease  escaping  or  resistance  to  disease. 

The  seed  of  the  particular  variety  itself  must  be  superior  in 
the  following: 

1.  Germinating  ability. 

2.  Good  color,  plumpness  and  weight. 

3.  Uniformity. 

4.  Freedom  from  diseases  transmitted  by  seed. 

5.  Freedom  from  any  other  damage. 

6.  Freedom  from  obnoxious  weeds. 

7.  Freedom  from  mixture  with  other  varieties. 

Adaptability. — We  have  already  indicated  that  no  one  variety 
always  excels  in  yield  or  quality.  All  that  the  experiment 
stations  can  do  is  to  determine  the  few  better  varieties  and  in  this 
way  assist  the  farmer  to  decide  which  to  grow. 

There  are  decided  advantages  in  limiting  the  number  of  varie- 
ties. It  is  of  considerable  value  for  one  locality  to  produce  large 
quantities  of  a  particular  variety.  Several  reasons  are  apparent, 
chief  of  which  are:  (1)  The  buyer  can  obtain  a  large  amount  of 
seed  of  that  particular  variety.  (2)  The  production  of  only  a 
few  varieties  or  a  single  variety  is  of  material  help  in  keeping 
purity  of  type,  as  there  is  not  so  much  opportunity  for  (a)  mix- 
tures in  thrashing,  growing,  etc.,  or  (b)  cross-fertilization  between 
varieties,  which  causes  variability  of  seed  and  plant  characters 
and,  therefore,  loss  of  purity  of  type. 

Yielding  Ability  and  Quality. — Variety  tests  carried  on  under 
experimentally  controlled  conditions  are  the  best  means  of  deter- 
mining comparative  yield  and  to  some  extent  comparative  quality 


FARMERS  METHODS  OF  PRODUCING  PURE  8EEDH     283 

of  different  strains.  Many  farmers  sustain  annual  losses,  which 
are  not  small,  due  to  using  seed  of  an  over-exploited  variety 
which  has  not  proved  its  worth  in  competitive  tests.  With 
many  crops,  quality  is  of  prime  importance  and  must  receive 
some  consideration  if  a  No.  1  grade  product  is  to  be  obtained. 

Purity. — For  crops  like  wheat,  oats,  and  barley,  which  are 
self-fertilized,  uniformity  is  the  rule,  providing  the  grower 
is  willing  to  pay  some  attention  to  eliminating  accidental  mix- 
tures. For  cross-fertilized  crops,  of  which  corn  is  a  good  example, 
purity  of  type  is  of  less  importance,  although  certain  general 
standards  of  purity  are  desirable. 

Hardiness. — Hardiness  is  a  feature  of  adaptability  but  it  de- 
serves especial  mention.  Ability  of  annual  crops  like  rye  and 
wheat  to  withstand  winter-killing  as  well  as  winter  hardiness 
for  perennial  crops  such  as  alfalfa  is  of  high  importance  and  is 
generally  given  much  consideration  by  experimenters  before 
recommending  a  particular  variety. 

Strength  of  Stalk. — Ability  to  stand  up,  which  obviates  injury 
from  lodging,  is  of  much  importance  in  grain  and  hay  crops.  In 
small  grains  early  lodging  often  causes  shriveled  seeds.  The 
difficulty  of  harvesting  is  greatly  increased  when  the  crop  is  flat. 

Disease  Escaping  or  Resistance. — Some  varieties  are  much 
freer  from  disease  than  others.  There  are  various  factors,  but  the 
chief  ones  may  be  considered  under  disease  escaping  and  disease 
resistance.  Disease  escaping  may  be  due  to  early  maturity,  as 
in  the  case  of  Marquis  wheat,  which  often  escapes  stem  rust  epi- 
demics when  late  varieties  such  as  Bluestem  are  seriousty  injured. 

Disease  resistance  is  the  condition  which  obtains  when  the 
organism  gains  entrance  to  the  plant  yet  causes  no  appreciable 
injury.  There  is,  for  example,  a  distinct  tendency  for  durum 
wheat  to  be  resistant  to  stem  rust;  some  durum  strains  being 
much  more  resistant  than  others. 

The  above  are  some  of  the  important  agronomic  or  horticultural 
characters  which  separate  one  variety  from  another.  By  a  knowl- 
edge of  these  the  grower  is  enabled  to  obtain  the  best  available 
strain  for  his  conditions.  Seed  of  this  selected  variety  must  then 
be  saved  in  such  a  manner  that  it  will  have  germinating  ability, 
i.e.,  will  grow  vigorously.  In  order  to  do  this  the  seed  must  be 
mature  and  well  developed  and  free  from  transmissible  diseases. 
Freedom  from  obnoxious  weed  seeds  is  also  an  important  con- 
sideration. 


284  BREEDING  CROP  PLANTS 

METHODS  OF  SEED  PRODUCTION 

After  obtaining  the  better  variety  for  the  locality,  the  seed 
grower  has  the  problem  of  keeping  this  variety  in  the  same  high 
state  of  production  and  if  possible  to  improve  it.  The  purpose 
of  this  chapter  is  to  outline  methods  for  the  various  crops  which 
may  be  used  by  the  seed  grower  or  by  the  average  farmer. 

Farm  crops  may  be  placed  in  four  groups  according  to  their 
modes  of  reproduction.  There  is  a  close  relation  between  this 
characteristic  and  the  farmer's  methods  of  seed  production. 
The  four  groups  mentioned  are  as  follows: 

Group  1. — Generally  self-fertilized:  Barley,  wheat,  oats,  peas,  beans,  flax, 
tobacco.  / 

Group  2. — Often  cross-pollinated:  Corn,  rye,  most  grasses,  root  crops. 

Group  3. — Cross-pollination  obligatory:  Red  clover,  sunflower. 

Group  4. — Vegetatively  propagated :  Potatoes,  sugar  cane,  sweet  potatoes. 

Among  farm  crops,  the  production  of  seed  generally  depends 
on  a  union  of  the  male  reproductive  cell,  contained  in  the  pollen 
grain,  with  the  female  reproductive  cell — the  egg  cell. 

The  pollen  grains  of  corn  are  produced  in  the  tassel  and  each 
thread  of  silk  leads  to  an  ovary  which  contains  the  egg  cell.  In 
order  to  produce  seed,  the  male  reproductive  cell  must  pass  down 
through  the  silk  and  unite  with  the  female  cell.  This  process  is 
called  fertilization.  If  pollen  and  silk  are  borne  by  the  same  plant 
the  process  is  self-fertilization,  and  if  by  different  plants,  cross-fer- 
tilization. As  the  egg  cell  and  the  pollen  grain  of  self-fertilized 
plants  are,  as  a  rule,  alike  in  their  inherited  characteristics,  the 
progeny  of  a  single  self-fertilized  plant,  such  as  barley,  wheat,  or 
oats,  have  the  same  inheritance.  There  is,  of  course,  considerable 
variation  in  all  characters,  owing  to  environmental  effect,  but  all 
evidence  shows  that  these  differences  are  not  truly  inherited. 
Occasional  crosses  occur  in  self-fertilized  crops  which  cause  inher- 
itable variability.  Mass  selection  serves  to  eliminate  these  off 
types. 

SEED  GROWERS  METHODS  FOR  SELF-FERTILIZED  PLANTS 

For  self-fertilized  plants  the  grower  can,  as  a  rule,  obtain  a 
pedigreed  strain  which  is  nearly  adapted  to  his  conditions.  The 
only  thing  that  he  can  do  with  this  variety  is  to  save  seed  in  such 
a  way  that  mixtures  of  other  strains  or  occasional  crosses  are 
eliminated,  together  with  obnoxious  weed  seeds  and  diseases. 


FARMERS'  METHODS  OF  PRODUCING  PURE  SEEDS     285 

The  strain  in  question  can  be  kept  in  a  pure  condition  for  its 
characters,  and  if  it  is  not  entirely  pure  at  the  outset  a  correct 
method  of  seed  selection  will  tend  to  purify  it  and  thus  to 
increase  its  value.  The  work  for  self-fertilized  crops  is  very 
simple  as  compared  with  the  production  of  improved  seed  of 
cross-fertilized  crops  or  the  production  of  highly  bred  livestock. 
For  self-fertilized  crops  the  method  outlined  is  essentially  that 
which  is  compulsory  for  the  production  of  registered  seed  by 
the  Canadian  Seed  Growers'  Association. 

The  steps  are  given  here  with  the  understanding  that  the  grower 
has  already  obtained  the  best  available  variety  for  his  soil  and 
climatic  conditions.  The  chief  points  are  as  follows: 

1.  The  use  of  a  yearly  hand-selected  seed  plot  of  at  least 
K    acre   in   size,    in   a   good    state   of    cultivation,    free   from 
weeds,  under  a  proper  rotation,  and  sown  at  the  regular  rate  of 
seeding. 

2.  The  hand  selection  from  this  plot  of  enough  seed  of  uniform 
character,  thoroughly  mature  and  free  from  disease,  to  plant  the 
following  year's  seed  plot.     This  selection  may  be  accomplished 
before  the  plot  is  harvested  or  from  the  shock  before  thrashing. 

3.  The  selected  heads,  panicles,  or  pods  should  be  thrashed  by 
hand  and  the  seed  carefully  stored. 

4.  The  removal  of  all  impurities,  weed  seeds  or  mixtures  of 
other  varieties,  from  the  seed  plot  before  it  is  harvested.     Purity 
of  seed  is  important. 

5.  The  bulk  crop  on  the  seed  plot  should  be  allowed  to  mature 
thoroughly,  should  be  harvested  carefully,  and  used  the  following 
year  to  sow  as  much  of  the  bulk  field  as  possible. 

According  to  plans  adopted  by  the  Canadian  Seed  Growers' 
Association,  seed  may  be  registered  which  is  not  more  than 
three  generations  away  from  the  hand-selected  seed  plot.  Such 
seed  is  inspected  in  the  field  and  after  being  thrashed,  and  must 
conform  to  certain  standards  of  purity  and  freedom  from  diseases. 

The  seed  plot  method  is  of  particular  interest  to  farmers  for 
grain  crops — barley,  wheat,  and  oats.  It  could  be  used  to 
advantage  for  flax,  beans,  and  possibly  peas,  although  in  the 
case  of  peas  the  selection  of  seed  would  be  somewhat  more 
difficult.  For  these  crops  there  seems  to  be  no  good  reason 
why  the  seed  plot  could  not  be  a  part  of  the  main  field,  although 
the  grower  must  not  forget  that  the  seed  plot  needs  some  extra 
attention  if  the  work  is  to  be  worth  while. 


286  BREEDING  CROP  PLANTS 

The  seed  plot  method  is  here  outlined  by  means  of  a  diagram. 

DIAGRAM  OF  FARMER'S   METHOD  OF    MAINTAINING  THE  PURITY  OF  SELF 

FERTILIZED  CROPS 


1st  year 

Field 

Hand  selected 
seed  plot 
M  A) 

H 
25-30  Ibs.     .     ^ 

and  selected 
seed  plot 

Seed  of  a  variety 
recommended  for 
the  locality  or 
•which  has  been 
grown  successfully 
in  the  locality 

25-30  Ibs.  of 
typical  heads 

Impurities 
removed 
before 
harvesting 

selected  by  hand 

selected  by  hand 

\ 

\ 

Canadians  register  seeds  as  1st,  2nd  or  3rd 
generation  seed  according  to  the  source  of  the 
seed  and  the  number  of  generations  away  from 
the  H.  S.  P.  (Hand  Selected  Seed  Plot.) 


For  the  tobacco  crop  there  is  no  necessity  of  a  seed  plot. 
The  grower  should  select  good-type  plants  in  the  field  and  save 
these  for  seed  production.  The  best  growers  insure  the  produc- 
tion of  self-fertilized  seed  by  covering  the  inflorescence  before  any 
of  the  flowers  open,  with  a  12-lb.  manila  paper  bag.  It  is 
necessary  to  remove  the  bag  from  time  to  time  to  shake  out  the 
dead  parts  of  the  corolla  so  that  the  seed  will  not  become  damaged. 
Ten  or  twelve  plants  handled  in  this  manner  furnish  sufficient 
seed  for  a  large  acreage. 

If  the  farmer  is  troubled  with  flax  wilt  he  can  easily  over- 
come this  difficulty  by  seed  selection.  All  that  is  necessary  is 
to  select  from  a  plot  on  which  the  wilt  disease  is  causing  con- 
siderable loss  those  plants  which  appear  to  be  free  from  the  dis- 
ease. Experiments  carried  on  by  Bolley  at  the  North  Dakota 
Station  which  have  been  recently  corroborated  (Stakman  et  al, 
1919),  have  shown  that  a  wilt-resistant  type  can  be  produced 
by  three  years  of  continuous  selection.  Methods  of  producing 
wilt  resistant  seed  are  presented  here  in  diagrammatical  form: — 


FARMERS'  METHODS  OF  PRODUCING  PURE  SEEDS     287 
DIAGRAM  OF  METHOD  OP  CONTROLLING  FLAX  WILT  BY  SELECTION 


Wilt  resistant 

1st  year 

Seed    Plot 

Select  enough  resistant   plants 

2d  year 

Seed    Plot 

Wilt    sick 

Wilt    sick 
soil 

seed 

soil 

by  hand  for  seed  plot  of 
following  year 

If  wilt  resistant  seed  is  not  available 
produce  it  by  selecting  plants  which  are 
resistant  under  wilt  conditions;  three  years 
of  continuous  selection  will  accomplish  this. 


Field 


IMPROVED  CORN  SEED 

The  determination  of  the  better  variety  of  corn  to  grow  is  not 
difficult.  The  farmer  can  obtain  reliable  advice  from  the  local 
county  agent  or  by  consulting  the  nearest  experiment  station. 
The  introduction  of  new  varieties  of  corn  from  other  states  before 
they  have  been  tested  for  the  climatic  conditions  in  question  is  a 
very  undesirable  practice  and  as  a  rule  a  cause  of  much  annual 
loss  to  the  corn  grower.  The  problem  with  corn  is  somewhat 
different  from  that  with  the  self-fertilized  crops.  Corn  is  cross- 
fertilized,  therefore  constant  inherited  variability  is  the  rule. 
When  a  variety  is  introduced  from  another  locality  it  undergoes 
a  process  of  selection  which  may  markedly  change  its  characters. 
Selection  in  a  pedigreed  line  of  wheat,  on  the  contrary,  does  not 
change  its  characters  and  serves  only  to  keep  the  variety  in  the 
same  state  of  purity  by  artificially  removing  any  possible  mix- 
tures which  may  occur.  This  brief  discussion  will  probably 
serve  to  show  that  seed  selection  on  the  farm  is  a  very  impor- 
tant practice  for  the  corn  grower,  unless  there  is  a  local  grower 
of  high  grade  seed. 

The  corn  seed  grower  faces  another  difficulty  which  the  small- 
grain  seed  producer  does  not  have  to  consider.  With  small 
grains — barley,  oats,  and  wheat — purity  for  all  characters  is  the 
general  rule.  This  has  led  the  corn  breeder  also  to  attempt 
to  obtain  purity  of  type.  Carefully  controlled  investigations 
have  served  to  show  a  possible  fallacy  in  this  practice.  The 


288 


BREEDING  CROP  PLANTS 


report  of  a  recent  study  at  the  Minnesota  Station  (Olson,  Bull,  and 
Hayes,  1918),  which  contains  experimental  evidence  together 
with  a  review  of  other  experiments  in  relation  to  score  card 
characters  and  yield,  show  no  correlations  between  individual 
characters  'such  as  trueness  to  the  ideal  score  card  ear  type  and 
subsequent  yield  of  these  ears. 

Artificial  self-fertilization  in  corn  isolates  homozygous  types 
which  are  less  vigorous  than  normally  cross-pollinated  plants. 
All  other  evidence  seems  to  show  that  too  close  a  purity  of 
type  corn  tends  to  a  reduction  in  vigor.  The  grower  whose 
method  of  selection  is  based  upon  ear  type  is  certainly  obtaining 
no  gain  in  yield  of  shelled  corn  per  acre.  The  detrimental 
results  of  too  close  selection  to  type  may  not  be  very  apparent  and 
may  be  more  than  counterbalanced  by  the  extra  attention  from  a 
cultural  standpoint,  for  an  interest  in  ideal  ear  types  certainly 
stimulates  the  farmer  to  produce  better  corn.  It  is  not,  however 
an  increase  due  to  better  breeding  but  to  better  cultural  practice. 

The  present  purpose  is  to  outline  methods  of  seed  selection.  As 
there  is  no  apparent  relation  between  score  card  characters  for 
type  of  ear  planted  (within  a  particular  variety)  and  resultant 
yield,  even  though  such  selection  may  be  constantly  practiced,  we 
may  pay  little  attention  to  those  characters  as  far  as  our  breeding 
plan  goes.  The  grower  should,  of  course,  produce  corn  of  one 
variety  which  is  pure,  judged  by  easily  evident  characters,  such  as 
color  of  seed  and  cob.  Abnormalities,  such  as  very  large  butts, 
badly  flattened  cobs,  or  very  irregularly  rowed  ears,  should  not 
be  used  as  foundation  stock.  Aside  from  these  there  is  no  need 
of  paying  much  attention  to  type.  Ability  of  a  variety  to 
mature  under  the  conditions,  is  very  important  and  needs  much 
attention. 

Two  methods  of  work  are  outlined  here,  either  of  which  may  be 
of  considerable  value  in  increasing  yield. 


METHOD  OF  BREEDING  CORN  FOR  SPECIAL  BREEDERS 

Nearly  all  discussions  of  corn  breeding  are  based  on  the  ear-to- 
row  method.  Such  a  method  takes  considerable  time  and  can  be 
carried  out  only  by  the  breeder  or  occasional  seed  specialist. 
The  ear-to  row  test  is  commonly  understood.  It  consists  of 
growing  the  seed  of  a  certain  number  of  ears  in  individual  rows 
and  determining  the  better  yielding  ones.  Each  ear  saved  is 


FARMERS'  METHODS  OF  PRODUCING  PURE  SEEDS     289 

then  a  basis  of  further  selection.  Complicated  methods  have 
been  used  for  the  introduction  of  new  blood  and  to  keep  up  the 
vigor  of  the  strain.  The  method  here  outlined  is  an  attempt 
to  simplify  this  practice  and  at  the  same  time  obtain  as  good 
results  as  can  be  obtained  by  the  more  detailed  procedures.  It 
is  based  on  experimental  studies  carried  on  at  the  Nebraska 
Station  (Montgomery,  1909).  The  details  are  as  follows: 

1.  Select  from  100  to  200  ears  of  the  variety  to  be  grown.     If 
possible,  select  these  ears  in  the  field  from  those  stalks  which  if 
in  a  perfect  stand  will  give  a  good  yield. 

2.  Make  an  ear-to-row  test  of  these  selected  ears,  saving  half 
of  the  seed  from  each  ear  planted.     From  this  ear-to-row  test 
the  25  best  ears  may  be  determined. 

3.  Mix  the  remnants  of  the  25  highest  yielding  ears  and  plant 
the  following  year  in  a  seed  plot.     Select  all  ears  obtained  which 
are  fairly  desirable,  eliminating  only  the  very  undesirable  types. 

4.  Use  the  selected  seed  for  planting  as  much  of  the  corn 
acreage  as  possible. 


DIAGRAM  OF  PROCEDURE  FOR  SPECIAL  CORN  BREEDER 


1st  Year 


2nd  Year 


3rd  Year 


4th  to 
3th  Year 


Use  remnants  of  25  best 
yielding  ears 


Save  seed 
in  fall 

Seed  Plot 
of  at  least 

from  per- 
fect stand 
hills  and 

1  acre 

V     vigorous 
\      stalks. 

Repeat  ear-to-row  test  at  the  end  of  the  8th 
year  and  proceed  as  before. 


Field 


U> 


290  BREEDING  CROP  PLANTS 

5.  Give  special  attention  to  a  part  of  the  field  so  that  a  uniform 
stand  may  be  obtained.     Select  enough  seed  from  this  part  of  the 
field  for  the  entire  acreage.     Select  seed  for  the  following  year's 
seed  plot  in  the  fall  before  a  killing  frost,  from  perfect  stand 
hills  and  from  those  stalks  which  appear  free  from  disease  and 
which  under    competition    show    ability    to    produce    one    or 
more  good  ears.     Throw  away  only  the  ears  of  very  undesirable 
type. 

6.  Continue  the  method  outlined  under  5  for  a  period  of  four 
or  five  years  and  then  use  again  the  ear-to-row  method  as  outlined 
under  1  and  2. 

METHOD  OF  CORN  BREEDING  FOR  AVERAGE  FARMER 

The  average  corn  grower  does  not  have  time  or  facilities  for 
accurate  ear-to-row  work.  The  method  here  outlined  is  very 
simple,  yet  is  probably  nearly  as  good  for  the  average  corn  variety 
as  the  more  complicated  one  previously  given. 

1.  (a)  Give  special  attention  to  a  part  of  the  field,  or  use  a  seed 
corn  plot. 

(6)  Plant  and  cultivate  carefully,  using  the  hill  method, 
and  grow  four  stalks  per  hill. 

(c)  Each  fall  before  frost  select  enough  seed  for  the  follow- 
ing year's  seed  plot  from  stalks  which  give  a  good  yield  and 
which  grow  in  four-stalk  hills. 

(d)  Discard  only  the  very  undesirable  ears  and  store  each 
selected  ear  in  a  careful  manner. 

(e)  Test  all  seed  used  for  germination. 

2.  Save  all  good  seed  produced  by  the  yearly  seed  plot  to 
plant  the  general  field. 

3.  Continue  1  and  2  each  crop  season. 

POTATO  SEED   (TUBERS)  SELECTION 

All  localities  are  not  equally  good  for  producing  potato  tubers 
for  planting,  therefore  it  will  be  better  for  some  farmers  to  buy 
tubers  from  a  different  locality.  At  University  Farm,  experi- 
ments of  the  Division  of  Horticulture  show  that  tubers  should 
be  produced  at  some  other  locality  if  high  yields  are  to  be  ob- 
tained. For  the  farmer,  however,  who  lives  in  a  locality  where 


FARMERS'  METHODS  OF  PRODUCING  PURE  SEEDS     291 


desirable  tubers  for  planting  are  produced,  there  are  some  methods 
which  are  of  help  to  him  in  saving  tubers. 

Ordinarily  the  plants  grown  from  tubers  of  a  single  plant 
are  alike  except  for  the  occasional  changes  which  occur  in  the 
inherited  characters  of  the  plant  itself.  Mixture  in  commercial 
tubers  is  one  common  cause  of  lack  of  purity  of  type.  The  selec- 
tion of  tubers,  therefore,  gives  the  grower  an  opportunity  to  im- 
prove his  variety  and  also  insures  a  constant  supply  of  tubers  free 
from  diseases.  This  freedom  from  disease  is  a  very  important 
point  (Tolaas  and  Bisby,  1919). 

The  first  step  of  the  grower  is  to  obtain  the  best  available 
variety,  true  to  type  and  free  from  disease.  After  obtaining 
such  a  variety,  one  of  the  following  plans  may  be  followed.  Both 
are  alike  for  the  first  year's  work. 

DIAGRAMMATICAL  ILLUSTRATION  OF  TUBER  SEED  PLOT  SELECTION  OF  THE 

POTATO 


4M  year 
4th  year 


1st  year               2d  year                 3d  year 

1st  year 

2  d  year                     3d 

year 

Dig  100 
bills 
by  hand 

Field 

Method  I 


Bulk 
seed 
plot 

Best 

Bulk 
seed 
plot 

/ 

Bulk, 
seed 
plot 

bills 

/ 

Etc. 


Method  11 


H  ill- 
to  -row 
seed  plot 

Best 

Hill- 
to-  row 
seed  plot 

strains 

292  BREEDING  CROP  PLANTS 

First  Year. — (a)  Remove  from  the  part  of  the  field  used  for 
saving  tubers  all  plants  which  show  evidences  of  diseases.  This 
should  be  done  during  the  growing  season. 

(b)  At  harvest  time  dig  at  least  100  hills  by  hand,  keeping 
each  hill  separate. 

(c)  Use  tubers  from  a  number  of  the  better  hills  for  the  stock 
plot  the  following  year. 

Second  Year. — Method  I. — Plant  all  good  tubers  from  previous 
year's  selection  of  best  hills  in  a  bulk  seed  plot.  Enough  tubers 
should  be  used  to  plant  about  Y±  acre.  This  requires  approxi- 
mately 5  bushels,  which  allows  some  tubers  to  be  discarded. 

Method  II. — This  is  the  hill-to-row  method.  In  order  to 
compare  the  productive  capacity  of  each  selected  hill  it  is  desir- 
able to  have  each  row  the  same  length  and  planted  from  the 
same  total  weight  of  potatoes.  All  of  the  progeny  of  some  hills 
will  be  discarded  this  second  year.  Those  that  give  a  good  yield 
and  are  desirable  in  other  ways  may  be  further  tested. 

Third  Year.  Method  I. — Continue  the  stock  plot  by  the  same 
means  as  used  in  Method  I  for  the  second  year's  work,  and  use  all 
good  tubers  produced  each  year  in  this  seed  plot  for  field  planting. 
This  work  may  be  continued  each  succeeding  season  by  the  same 
plan. 

Method  II. — Make  a  further  test  of  the  best  selections  as 
determined  by  the  second  year's  test,  growing  much  longer  rows, 
thus  obtaining  more  reliable  results.  All  tubers  free  from  disease, 
of  the  best  yielding  strain  or  strains,  may  be  used  to  increase 
the  stock  the  following  year. 

The  essential  features  of  these  two  methods  are  presented  on 
page  291  in  diagrammatical  form.  Method  II  probably  is 
somewhat  better  if  all  details  of  the  test  are  carefully  performed. 
For  the  average  farmer,  Method  I  is  less  cumbersome  and  if 
constantly  practiced  would  probably  give  about  as  good  a  result 
as  Method  II. 

IMPROVEMENT  BY   SELECTION  OF  SUCH  CROPS  AS    ALFALFA, 
CLOVER,  AND  GRASSES 

Obtain,  if  possible,  a  variety  which  is  especially  adapted  to  the 
conditions.  Breeding  work  should  aim  at  producing  a  variety 
which  excels  in  resistance  to  winter  injury  and  to  plant  diseases 
and  is  also  a  high  producer  of  hay  and  seed. 


FARMERS'  METHODS  OF  PRODUCING  PURE  SEEDS     293 

The  following  is  an  outline  of  the  possible  steps : 

1.  Obtain  3  or  4  pounds  of  the  best  available  seed. 

2.  Plant  in  a  seed  plot  isolated  as  far  as  possible  from  other  crops  of  a 
like  kind.     Plant  seed  in  rows  3  ft.  apart  and  plant  two  or  three  seeds  in 
each  hill,  spacing  the  hills  two  ft.  apart  in  the  row. 

3.  Remove  all  but  a  single  plant  from  each  hill  when  the  plants  are  well 
started. 

4.  Keep  the  plot  free  from  weeds. 

5.  Discard  all  weak  plants  from  time  to  time  as  they  become  apparent. 

6.  Save  seed  of  all  desirable  plants  and  increase. 

The  improvement  of  the  class  of  crops  here  mentioned  is 
somewhat  more  difficult  than  with  small  grains,  corn,  and  pota- 
toes, and  should  be  undertaken  only  by  the  few  seed  producers 
who  are  willing  to  take  the  necessary  trouble  to  carry  out  care- 
fully the  details  as  outlined.  Controlled  experiments  at  some  of 
the  state  experiment  stations  and  in  Europe  have  shown  that 
much  gain  can  be  obtained  by  such  selection. 

SEED  REGISTRY  OR  CERTIFICATION 

The  outlined  seed  plot  methods  are  based  upon  fundamental 
breeding  principles.  In  order  to  protect  the  seed  grower  who 
follows  such  a  practice,  some  system  of  seed  certification  is 
advisable.  Various  methods  have  been  developed  by  crop 
improvement  associations.  The  details  of  procedure  are  those 
which  are  based  upon  fundamentally  sound  business  practice. 
Seed  that  is  eligible  for  registration  must  conform  to  certain 
standards  of  purity  and  freedom  from  plant  pests.  The  seed-plot 
methods,  if  carefully  followed,  insure  the  production  of  seed  of 
a  certain  standard  grade.  Certification  or  registration  shows 
that  the  seed  has  been  approved  by  the  trained  seed  inspector. 


DEFINITIONS1 

Acquired  Character. — A  modification  of  bodily  structure,  function, 
or  habit  which  is  impressed  on  the  organism  in  the  course  of  individual  life. 

Aieurone. — The  outermost  layer  of  the  endosperm  in  cereals,  when 
it  is  rich  in  gluten. 

Allelomorph. — One  of  a  pair  of  contrasted  characters  which  are  alternative 
to  each  other  in  Mendelian  inheritance.  Often  used,  but  with  doubtful 
propriety,  as  a  synonym  for  gene,  factor,  or  determiner. 

Allelomorphism. — A  relation  between  two  characters,  such  that  the 
determiners  of  both  do  not  enter  the  same  gamete  but  are  separated  into 
sister  gametes. 

Alternative  Inheritance. — A  distribution  of  contrasting  parental  or 
ancestral  characters  among  offspring  or  descendants,  such  that  the  individ- 
uals exhibit  one  or  other  of  the  characters  in  question,  combinations  or 
blends  of  these  characters  being  absent  or  exceptional. 

Anthesis. — The  period  or  act  of  flowering. 

Awn. — A  bristle-shaped  elongated  appendage  or  extension,  to  a  glume, 
akene,  anther,  etc. 

Barbed. — Furnished  with  rigid  points  or  short  bristles,  usually  reflexed. 

Biotype. — A  group  of  individuals  all  of  which  have  the  same  genotype. 

Bran.— The  coat  of  the  caryopsis,  consisting  of  pericarp  and  seed-coat 
united. 

Caryopsis. — A  one-seeded  dry  fruit  with  the  thin  pericarp  adherent  to  the 
seed,  as  in  most  grasses. 

Centgener. — Originally  used  by  W.  M.  Hays,  at  the  Minnesota  Station, 
to  refer  to  a  100-plant  plot  in  which  each  seed  was  planted  a  certain  distance 
from  each  other  seed. 

Chaff. — The  floral  parts  of  cereals,  generally  separated  from  the  grain  in 
thrashing  or  winnowing. 

Chimera. — An  association  of  tissues  of  different  parental  origin  and  genetic 
constitution  in  the  same  part  of  a  plant. 

Chromosome  hypothesis. — The  hypothesis  advanced  by  Morgan  in  which 
factors  are  arranged  in  the  chromosomes. 

Class. — In  genetics  a  group  that  includes  variates  of  similar  magnitude. 

Clone. — A  group  of  individuals  produced  from  a  single  original  individual 
by  some  process  of  asexual  reproduction,  such  as  division,  budding,  slipping, 
grafting,  parthenogenesis  (when  unaccompanied  by  a  reduction  of  the 
chromosomes),  etc. 

Coefficient  of  Variability. — A  relative  index  of  variation  obtained  by 
expressing  the  standard  deviation  in  percentage  of  the  mean. 

Coupling. — Such  a  relation  between  the  genes  of  two  unit-characters 
that  they  have  a  more  or  less  marked  tendency  to  be  included  in  the  same 
gamete  when  the  individual  is  heterozygous  for  both  of  the  genes  in  question. 

1  Many  of  the  genetic  definitions  are  taken  from  Shull  (1915),  Babcock 
and  Clausen  (1918)  or  others.  Ball  and  Piper's  (1916)  papers  on  termi- 
nology have  been  used  for  agronomical  terms. 

294 


DEFINITIONS  295 

Cross. — Synonymous  with  hybrid. 

Cross-fertilization. — The  union  of  the  egg  cell  of  an  individual  with  the 
sperm  cell  of  a  different  individual  whether  the  organisms  belong  to  the  same 
or  different  genotypes. 

Cross-over. — A  separation  into  different  gametes,  of  determiners  that 
are  usually  coupled,  and  the. association  of  determiners  in  the  same  gamete 
which  are  generally  in  different  gametes. 

Detassel. — To  remove  the  tassel,  as  in  maize. 

Cryptomere. — A  factor  or  gene  whose  presence  can  not  be  inferred  from 
an  inspection  of  the  individual,  but  whose  existence  can  be  demonstrated 
by  means  of  suitable  crosses. 

Determiner. — Synonymous  with  gene  or  with  factor  as  applied  in  genetics. 

Dominance.— In  Mendelian  hybrids  the  capacity  of  a  character  which 
is  derived  from  only  one  of  the  two  generating  gametes  to  develop  to  an 
extent  nearly  or  quite  equal  to  that  exhibited  by  an  individual  which  has 
derived  the  same  character  from  both  of  the  generating  gametes.  In  the 
absence  of  dominance  the  given  character  of  the  hybrid  usually  presents 
a  "blend"  or  intermediate  condition  between  the  two  parents,  but  may 
present  new  features  not  found  in  either  parent. 

Dominant. — (1)  A  character  which  exhibits  dominance,  i.e.,  that  one  of 
two  contrasted  parental  characters  which  appears  in  the  individuals  of  the 
first  hybrid  generation  to  the  exclusion  of  the  alternative  "recessive" 
character.  (2)  An  individual  possessing  a  dominant  character  in  contrast  to 
those  individuals  which  lack  that  character  which  are  called  "recessives." 

Ear. — A  large,  dense  or  heavy  spike  or  spikelike  inflorescence  as  the  ear  of 
maize.  Popularly  applied  also  to  the  spike-like  panicle  of  such  grasses 
as  wheat,  barley,  timothy  and  rye. 

Emasculation. — The  act  of  removing  the  anthers  from  a  flower. 

Endosperm. — The  substance  which  surrounds  the  embryo  in  many  seeds, 
as  the  starchy  part  of  a  kernel  of  wheat  or  corn. 

Factor. — An  independently  inheritable  element  of  the  genotype  whose 
presence  makes  possible  a  specific  reaction  or  the  development  of  a  particular 
unit-character  of  the  organism  which  possesses  that  genotype;  a  gene  or 
determiner. 

Floret. — A  small  flower,  especially  one  of  an  inflorescence,  as  in  grasses 
and  Compositse. 

Fi,  F2,  F?,  etc. — 1st,  2nd,  and  3rd,  etc.  generations  following  a  cross. 

Gamete. — A  reproductive  cell  containing  x  number  of  chromosomes. 

Gene. — Synonymous  with  determiner  or  factor. 

Genotype. — The  fundamental  hereditary  constitution  or  sum  of  all 
the  genes  of  an  organism. 

Glabrous. — Smooth,  especially  without  hairs. 

Glume. — One  of  the  two  empty  chaffy  bracts  at  the  base  of  each  spikelet 
in  grasses. 

Grain. — Cereal  seeds  in  bulk. 

Group. — In  genetics  a  broad  general  term  for  a  complex  of  other 
categories  and  not  for  a  complex  of  any  particular  category. 

Head. — A  dense,  short  cluster  of  sessile  or  nearly  sessile  flowers  on  a  very 
short  axis  or  receptacle,  as  in  red  clover  or  sunflower. 


296  BREEDING  CROP  PLANTS 

Heredity.— The  distribution  of  genotypic  elements  of  ancestors  among 
the  descendants;  the  resemblance  of  an  organism  to  its  parents  and  other 
ancestors  with  respect  to  genotypic  constitution. 

Heterozygosity. — The  condition  of  an  organism  due  to  the  fact  that  it  is  a 
heterozygote;  the  state  of  being  heterozygous;  the  extent  to  which  an  indivi- 
dual is  heterozygous. 

Heterozygote. — A  zygotic  individual  in  which  any  given  genetic  factor 
has  been  derived  from  only  one  of  the  two  generating  gametes.  Both  eggs 
and  sperms  produced  by  such  an  individual  are  typically  of  two  kinds,  half 
of  them  containing  the  gene  in  question,  the  rest  lacking  this  gene;  conse- 
quently the  offspring  of  heterozygotes  usually  consist  of  a  diversity  of 
individuals,  some  of  which  possess  the  corresponding  character  while  others 
lack  it. 

Heterozygous. — The  state  or  condition  found  in  a  heterozygote. 

Heterosis. — The  increased  growth  stimulus  often  exhibited  in  the  Fj 
generation  of  a  cross. 

Homozygosis. — The  state  of  being  homozygous;  the  extent  to  which  an 
individual  is  homozygous. 

Homozygote. — An  individual  in  which  any  given  genetic  factor  is  doubly 
present,  due  usually  to  the  fact  that  the  two  gametes  which  gave  rise  to  this 
individual  were  alike  with  respect  to  the  determiner  in  question.  Such  an 
individual,  having  been  formed  by  the  union  of  like  gametes,  in  turn  gener- 
ally produces  gametes  of  only  one  kind  with  respect  to  the  given  character, 
thus  giving  rise  to  offspring  which  are,  in  this  regard,  like  the  parents; 
in  other  words,  homozygotes  usually  "breed  true."  A"  positive  "homozy- 
gote  with  respect  to  any  character  contains  a  pair  of  determiners  for  that 
character,  while  a  "negative"  homozygote  lacks  this  pair  of  determiners. 

Homozygous. — The  state  or  condition  found  in  a  homozygote. 

Hybrids. — The  progeny  of  a  cross-fertilization  of  parents  belonging  to 
different  genotypes. 

Hull. — A  term  applied  to  include  the  lemma  and  palea  when  they  remain 
attached  to  the  caryopsis  after  thrashing. 

Hypostasis. — That  relation  of  a  gene  in  which  its  usual  reaction  fails  to 
appear  because  of  the  masking  or  inhibitory  effect  of  another  gene;  con- 
trasted with  "epistasis." 

Inflorescence. — The  flowering  part  of  a  plant. 

Keel. — A  central  ridge  resembling  the  keel  of  a  boat,  as  in  the  glumes  of 
some  grasses,  etc. ;  also  the  inferior  petal  in  the  legume  flowers. 

Kernel. — Matured  body  of  an  ovule;  seed  minus  its  coats. 

Lethal. — A  genetic  condition  causing  death. 

Linkage. — The  type  of  inheritance  in  which  the  factors  tend  to  remain 
together  in  the  general  process  of  segregation. 

Lodicule. — A  minute  scale  at  the  base  of  the  ovary  opposite  the  palea  in 
grasses,  usually  two  in  number,  and  probably  representing  the  reduced 
perianth. 

Mean. — The  arithmetical  average. 

Mode. — The  class  of  greatest  frequency. 

Mendelize. — To  follow  Mendel's  law  of  inheritance. 

Multiple  Allelomorphs. — Three  or  more  characters  which  are  so  related 
that  they  are  mutually  allelomorphic  in  inheritance. 


DEFINITIONS  297 

Mutant. — An  individual  possessing  a  genotypic  character  differing  from 
that  of  its  parent  or  those  of  its  parents,  and  not  derived  from  them  by  a 
normal  process  of  segregation. 

Mutate. — To  undergo  a  change  in  genotypic  character  independently  of 
normal  segregation. 

Ovule. — Female  sex  cell  with  its  immediate  surrounding  parts. 

Ovum. — Egg  cell. 

PI,  Po,  etc. — The  1st,  2nd,  etc.  generation  of  the  parents. 

Palea. — The  upper  of  the  two  bracts  immediately  enclosing  each  floret  in 
grasses. 

Panicle. — A  compound  inflorescence  with  pediceled  flowers  usually  loose 
and  irregular,  as  in  oats,  rye,  proso,  etc. 

Pedicel. — A  stalk  on  which  an  individual  blossom  is  borne. 

Peduncle. — The  primary  stalk  supporting  either  an  inflorescence  or  a 
solitary  flower.  In  grasses  the  uppermost  internode  of  the  culm. 

Pericarp. — The  matured  wall  of  the  ovary. 

Phenotype. — The  apparent  type  of  an  individual  or  group  of  individuals, 
i.e.  the  sum  of  the  externally  obvious  characteristics  which  an  individual 
possesses,  or  which  a  group  of  individuals  possess  in  common;  contrasted 
with  genotype. 

Presence  and  Absence  Hypothesis. — The  hypothesis  that  any  simple 
Mendelian  difference  between  individuals ,  results  solely  from  the  presence 
of  a  factor  in  the  genotype  of  the  one  individual,  which  is  absent  from  that  of 
the  other.  Presence  and  absence  of  unit-differences  as  a  convenient  method 
of  describing  the  results  of  genetic  experiments  should  be  carefully  distin- 
guished from  the  presence  and  absence  hypothesis.  The  method  is  purely 
objective  and  entirely  free  from  hypothetical  implications. 

Probable  Error. — A  measure  of  accuracy  for  results  obtained  by  statistical 
methods.  The  chances  are  even  that  the  true  value  lies  within  the  limits 
marked  by  the  probable  error. 

Probable  Error  of  a  Single  Determination. — S.  D.  X   ±0.6745. 

Probable  Error  of  a  Difference. — The  square  root  of  the  sum  of  the 
squares  of  probable  errors  of  the  two  results,  or  the  probable  error  of  a 
single  determination  multiplied  by  the  \/2. 

Pubescent. — Hairy  in  a  general  sense;  in  special  use,  covered  with  short, 
soft  hairs. 

Pure  Line. — A  group  of  individuals  derived  solely  by  one  or  more  self- 
fertilizations  from  a  common  homozygous  ancestor.  Sometimes  erroneously 
applied  to  groups  of  individuals  believed  to  be  genotypically  homogene- 
ous (a  homozygous  biotype  or  a  clone)  without  regard  to  the  method  of 
reproduction. 

Recombination. — Union  of  parental  factors  in  individuals  of  the  second  or 
later  generations  after  a  cross. 

Reduction  Division. — That  in  which  homologous  chromosomes  separate 
preparatory  to  formation  of  gametes. 

Repulsion. — Such  a  relation  between  two  genetic  factors  that  both  are 
not,  as  a  rule,  included  in  the  same  gamete,  referring  especially  to  cases  in 
which  the  factors  in  question  give  rise  to  obviously  different  characteristics; 
also  called  "spurious  allelomorphism." 


298  BREEDING  CROP  PLANTS 

Replication.— Systematic  repetition.  Used  in  field  work  to  designate  the 
systematic  distribution  of  plots  of  each  strain  or  variety  to  overcome  soil 
heterogeneity.  Two  replications  means  the  use  of  three  plots  systemati- 
cally distributed. 

Roguing. — The  act  of  removing  undesirable  individuals  from  a  varietal 
mixture  in  the  field  by  hand  selection. 

Seed. — -The  mature  ovule,  consisting  of  the  kernel  and  its  proper  coat. 

Self-fertilization. — The  union  of  the  egg  cell  of  one  individual  with  the 
sperm  cell  of  the  same  individual. 

Self -sterility. — That  condition  in  which  the  male  gametes  of  an  organism 
are  incapable  of  fertilizing  the  female  gametes  of  the  same  individual. 

Segregate. — With  reference  to  Mendelian  unit-characters,  to  become 
separated  through  the  independent  distribution  of  the  genetic  factors 
before  or  at  the  time  of  the  formation  of  the  gametes. 

Sex-linked  Inheritance. — The  association  of  the  determiner  for  any  unit- 
character  with  a  sex-determiner,  in  such  a  manner  that  the  two  determiners 
are  either  generally  included  in  the  same  gamete,  or  that  they  are  generally 
included  in  different  gametes. 

Somatic  Segregation. — Segregation  during  somatic  division. 

Species. — A  group  of  varieties  or  a  single  variety  which  in  botanical 
characters  and  genetic  relationship  can  be  differentiated  from  another  group 
or  variety  belonging  to  the  same  genus  or  to  other  genera. 

Spikelet. — A  small  or  secondary  spike,  especially  in  the  inflorescence  of 
grasses. 

Spike. — A  simple  inflorescence  with  the  flowers  sessile  or  nearly  so  on  a 
more  or  less  elongated  common  axis  or  rachis. 

Sperm  or  Sperm  Cell. — Male  sex-cell. 

Standard  Deviation. — An  absolute  measurement  of  variation  in  terms  of 
the  mean.  The  square  root  of  the  sum  of  the  deviations  squared  divided 
by  the  number  of  variates. 

Sterility. — Inability  to  reproduce;  when  male  and  female  gametes,  through 
incompatibility  or  some  other  cause,  are  incapable  of  mating  or  fertilization. 

Strain. — A  group  within  a  variety  which  constantly  differs  in  genetic 
factors  or  a  single  genetic  factor  difference  from  other  strains  of  the  same 
variety. 

Tassel. — Used  to  designate  the  staminate  inflorescence  of  maize. 

Unit-character. — In  Mendelian  inheritance,  a  character  or  alternative 
difference  of  any  kind,  which  is  either  present  or  absent,  as  a  whole,  in  each 
individual,  and  which  is  capable  of  becoming  associated  in  new  combinations 
with  other  unit-characters. 

Variate. — -A  single  magnitude  determination  of  a  character. 

Variety. — A  group  of  strains  or  a  single  strain  which  by  its  structural  or 
functional  characters  can  be  differentiated  from  another  variety. 

Variety  Group. — A  complex  of  varieties  which  resemble  each  other  more 
than  varieties  belonging  to  a  different  group.  Of  lower  grade  than  species. 

Xenia. — The  apparent  immediate  effect  of  pollen.  It  results  from 
double  fertilization. 

Zygote. — The  body  formed  by  the  union  of  two  gametes  and  containing 
2x  number  of  chromosomes. 


LITERATURE  CITATIONS 

AARONSOHN,  A.,  1910.     Agricultural  and  botanical  explorations  in  Palestine. 

U.  S.  Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  180,  52  pp. 
ALLARD,  H.  A.,  1919a.     Some  studies  in  blossom  color  inheritance  in  tobacco, 

with  special  reference  to  N.  sylvestris  and  N.   tabacum.     Am.  Nat., 

63 :  79-84. 

19196.    Gigantism  in  Nicotiana  tabacum  and  its  alternative  inheritance. 

Am  Nat.,  53  :  218-233. 
ALKEMINE,  M.,  1914.     tlber  das  Bliihen  des  Reises  und  einige  sich  daran 

anknupf ende  Erscheinungen.     Zeitschr.  f iir  Pflanzenziicht. ,  2 : 339-375. 
ANONYMOUS,    1919.     New   Iowa   oats   and   the    man   who   found   them. 

Wallace's  Farmer,  44  :  771. 
ANTHONY,  R.  D.,  and  HEDRICK,  U.  P.,  1916.     Some  notes  on  the  breeding  of 

raspberries.     New  York  (Geneva)  Agr.  Exp.  Sta.,  Bull.  417:  75-88. 
ARNY,  A.  C.,  and  HAYES,  H.  K.,  1918.     Experiments  in  field  technic  in  plot 

tests.     Jour.  Agr.  Res.,  15:  251-262. 
ARNY  A.  C.  and  GARBER,  R.  J.,  1918.     Variation  and  correlation  in  wheat, 

with  special  reference  to  weight  of  seeds   planted.     Jour.  Agr.  Res., 

14 :  359-392. 

BABCOCK  E.  B.  and  CLAUSEN,  R.  E.,  1918.     Genetics  in  relation  to  agricul- 
ture.    McGraw-Hill  Book  Company,  New  York,  675  pp. 
BACKHOUSE,  W.  O.,  1916-17.     Note  on  the  inheritance  of  "crossability." 

Jour.  Genetics,  6 :  91-94. 

1918.     The  inheritance  of  glume  length  in  T.  polonicum.     A  case  of 

zygotic  inhibition.     Jour.  Genetics,  7 :  125-135. 
BAILEY,  L.  H.,  1898.     "Evolution  of  Our  Native  Fruits."     The  Macmillan 

Co.,  London,  472  pp. 

1890.     Experiences  in  crossing     cucurbits.     Ann.  Rept.  Cornell  Agr. 

Exp.  Sta.,  3 :  180-187. 

1900.     Cyclopedia  of  American   horticulture.     The   Macmillan   Co., 

New  York,  I,  410  pp. 

1909.  Cyclopedia  of  American  agriculture.     Vol.   2.     Crops.     The 
Macmillan  Co.,  New  York,  699  pp. 

BAIN,  M.,  and  ESSARY,  S.  H.,  1906.     Selection  for  disease-resistant  clover. 

A  preliminary  report.     Tenn.  Agr.  Exp.  Sta.,  Bull.  75,  10  pp. 
BALL,  C.  R.,  1910.     The  history  and  distribution  of  sorghum.     U.  S.  Dept. 

Agr.  Bur.  Plant  Indust.,  Bull.  175,     63  pp. 

1910.  The  breeding  of  grain  sorghums.     Amer.   Breeders'  Mag.,  1: 
283-293. 

1911.  The   importance   and   improvement   of   the   grain   sorghums. 
U.  S.  Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  203,  42  pp. 

BALL,  C.  R.  and  Piper,  C.  P.,  1916.  Contributions  to  agronomic 
terminology.  Jour.  Amer.  Soc.  Agron.,  8:  1-9,  197-204,  228-237, 
310-315. 

299 


300  BREEDING  CROP  PLANTS 

BALLS,   W.  L.,    1908.     Mendelian  studies  of  Egyptian  cotton.     Jour,  of 
Agr.  Sci.,  2 :  346-379. 

1911    (1913).     The   inheritance   of   measurable  characters  in  hybrids 
between  reputed  species  of  cotton.     Int.  Conf .  de  Genetique,  4 : 429-440. 

1912.  The   cotton  plant  in  Egypt,   studies  in  physiology  and  gene- 
tics.    The  Macmillan  Co.,  London,  202  pp. 

BARBER,  C.  W.,  1914.     Note  on  the  influence  of  shape  and  size  of  plots  in 

tests  of  varieties  of  grain.     In  Maine  Agr.  Exp.  Sta.,  Bull.  226:  76-84. 
BARRUS,  M.  F.,  1918.     Varietal  susceptibility  of  beans  to  strains  of  Colleto- 

trichum      lindemuthlanum    (Sacc.   &  Magn.)    B.  &  C.     Phytopath., 

8:589-614. 
BATCHELOR,  L.  D.,  and  REED,  H.  S.,  1918.     Relation  of  the  variability  of 

yields  of  fruit  trees  to  the  accuracy  of  field  trials.     Jour.  Agr.  Res.,  12: 
245-283. 
BAUR,    E.,    1914.     Einfiihrung    in    die    experimentelle    Vererbungslehre. 

Geb ruder  Borntrseger.      Berlin  (2nd  edition).     401  pp. 
BEACH,  S.  A.,  1898.     Self-fertility  of  the  grape.     New  York  State  Exp.  Sta., 

Bull.  157:  397-441. 

1899.     Fertilizing   self-sterile   grapes.     New    York    State    Exp.  6ta., 

Bull.  169:  331-371. 

1902.     Investigations  concerning  the  self-fertility  of  the  grape.     New 

York  State  Exp.  Sta.,  Bull.  223:  269-290. 
BEAL,  W.  J.,  1876-1882.     Reports  Michigan  Board  of  Agriculture  1876, 

1877,  1881  and  1882. 
BELLING,  J.,  1912o.     Third  generation  of  the  cross  between  velvet  and  Lyon 

beans.     Rpt.  of  Florida  Agr.  Exp.  Sta.  for  1912:  115-127. 

19126.     Crossing    Corn.     Florida    Agr.    Exp.    Sta.    Press   Bull.    97. 

1913.  Report   of   Assistant  Botanist.     Rpt.   of   Florida   Agr.  Exp. 
Sta.  for  1913 :  104-130. 

1914o.     The  mode  of  inheritance  of  semi-sterility  in  the  offspring  of 

certain  hybrid  plants.     Zeitschr.  fur  Ind.  Abstamm.  u.  Vererb  ,  12 : 

303-342. 

19146.     Inheritance  of  pod  pubescence  and  partial  sterility  in  Stizolob- 

ium  crosses.     Rpt.  of  Florida  Agr.  Exp.  Sta.  for  1914 :  31-55. 

1915«.     Inheritance  of  mottling  of  the  seed-coat.     Rpt.  of  Florida  Agr. 

Exp.  Sta.  for  1915:  111-127. 

19156.     Inheritance  of  length  of  pod  in  certain  crosses.     Jour.  Agr. 

Res.,  6:405-420. 
BERTHAULT,    P.,    1911  (1913).     Note  Preliminaire  sur  1'origine  specifique 

de  la  Pomme  de  Terre.     Int.  Conf.  de  Genetique,  4 :  377-380. 
BIFFEN,  R.  H.,   1905.     Mendel's  law  of  inheritance  and  wheat  breeding. 

Jour.  Agr.  Sci.,  1:  4-48. 

1907a.     Studies  in  the  inheritance  of  disease  resistance.     Jour.  Agr. 

Sci.,  2:109-128. 

19076.     The  hybridization  of  barleys.     Jour.  Agr.  Sci.,  2 : 183-206. 

1912.     Studies   in    the  inheritance  of  disease  resistance.     II.    Jour. 

Agr.  Sci.,  4:421-429. 

1916.     The  suppression  of  characters  on  crossing.     Jour.  Genetics,  6  : 

225-228. 


LITERATURE  CITATIONS  301 

1917.     Systematized  plant  breeding.     In  Seward's  "Science  and  the 

Nation,"  pp.  146-175.     University  Press,  Cambridge. 
BOLLEY,  H.  L.,  1901.     Flax  wilt  and  flax  sicksoil.     North  Dakota  Agr.  Exp. 

Sta.,  Bull.  50:  27-57. 

1903.     Flax  and  flax  seed  selection.     North  Dakota  Agr.  Exp.  Sta., 

Bull.  55:  171-198. 

1909.     Some  results  and  observations  noted  in  breeding  cereals  in  a 

specially  prepared  disease  garden.     Proc.  Amer.  Breeders'  Assoc.,  5: 

177-182. 
BRAND,  C.  J.,  1911.     Grimm  alfalfa  and  its  utilization  in  the  Northwest. 

U.  S.  Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  209,  66  pp. 
BURKHOLDER,  W.  H.,  1918.     The  production  of  an  anthracnose-resistant 

white  marrow  bean.     Phytopath.    8 :  353-359. 
CAPORN,  A.  ST.  CLAIR,  1918.     An  account  of  an  experiment  to  determine  the 

heredity  of  early  and  late  ripening  in  an  oat  cross.     Jour.  Genetics, 

7 :  247  -  257. 
CARLETON,    M.  A.,  1916.     The  small  grains.     The    Macmillan   Co.,   New 

York,  699  pp. 
CARRIER,  LYMAN,   1919.     A  reason  for  the  contradictory  results  in  corn 

experiments.     Jour.  Amer.  Soc.  Agron.,  11:  106-113. 
COE,  H.  S.,  1918.     Origin  of  the  Georgia  and  Alabama  varieties  of  velvet 

bean.     Jour.  Amer.  Soc.  Agron.,  10:  175-179. 
COLLINS,  G.  N.,  1909.     The  importance  of  broad  breeding  in  corn.     U.  S. 

Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  141:  31-44. 

1910a.     The  value  of  first-generation  hybrids  in  corn.     U.  S.  Dept.  Agr., 

Bur.  Plant  Indust.,  Bull.  191,  45  pp. 

19106.     Increased   yields   of   corn   from  hybrid   seed.     Yearbook  of 

U.  S.  Dept.  of  Agr.  for  1910 :  319-328. 

1912.  The    origin  of   maize.     Jour.  Washington   Acad.    of   Sci.,    2 : 
520-530. 

1917.     Hybrids  of  Zea  ramosa  and    Zea  tunicata.     Jour.  Agr.  Res., 
9 :  383-395. 

COLLINS,  G.  N.  and  KEMPTON,  J.  H.,   1911.      Inheritance  of   waxy  endo- 
sperm in  hybrids  of  Chinese  maize.     Int.  Conf .  de  Genetique,  4 :  347-357. 

1913.  Effects  of  cross-pollination  on  the  size  of  seed  in  maize.     In 
Misc.  Papers,  U.  S.  Dept.  Agr.,  Bur.  Plant  Indust.,  Cir.  124. 

1914.  Inheritance  of  endosperm  texture  in    sweet  X  waxy  hybrids 
of  maize.     Amer.  Nat.,  48 :  584-594. 

COOK,  O.  F.     1913.     Heredity  and  cotton  breeding.     U.  S.   Dept.  Agr., 
Bur.  Plant  Indust.,  Bull.  256,  113  pp. 

1915.  Brachysm,  a  hereditary  deformity  of  cotton  and  other  plants. 
Jour.  Agr.  Res.,  3 :  387-400. 

CORRENS,  C.,  1899.     Untersuchungen   iiber  die   Xenien   bei   Zea    Mays. 

Ber.  d.  Deutsch.  Bot.  Ges.,  17:  410-417. 

1901.    Bastarde  zwischen  Maisrassen  mit  besonderer  Beriicksichtigung 

der  Xenien.     Bibliotheca  Botanica,  53 :  1-161. 
CRANE,    M.   B.,   1915.     Heredity  of  types  of  inflorescence  and  fruits  in 

tomato.     Jour.  Genetics,  5:  1-11. 
CRANDALL,  C.  S.,  1918.     Apple-bud  selection,  apple  seedlings  from  selected 

trees.     Illinois  Agr.  Exp.  Sta.,  Bull.  211:  181-264. 


302  BREEDING  CROP  PLANTS 

CUMMINGS,   M.   B.,   1904.     Fertilization  problems:  A  study  of  reciprocal 

crosses.     Maine  Agr.  Exp.  Sta.  Bull.,  104:  81-99. 
CUTLER,   G.   H.,    1919.     A  dwarf  wheat.     Jour.    Amer.  Soc.  Agron.,  11: 

76-78. 
DARWIN,  CHARLES,  1877.     The  effects  of  cross-  and  self-fertilization  in  the 

vegetable  kingdom.     Appleton  &  Co.,  London. 
DE    CANDOLLE,    ALPHONSE,    1886.     Origin   of  cultivated   plants.     Kegan 

Paul,  Trench  &  Co.,  London,  468  pp. 

DETTWEILER,  1914.     Aryan  agriculture.     Jour.  Heredity,  6:  473-481. 
DE  VRIES,  HUGO,  1899.    Sur  la  fecondation  hybride  de  1'albumen.    Comptes 

Rend,  des  Seances  de  1' Academe  des  Seances,  129 :  973-975. 

1906.  Species     and     varieties:    their     origin    by    mutation.     Open 
Court  Pub.  Co.,  Chicago. 

1907.  Plant  breeding.     Open  Court  Pub.  Co.,  Chicago,  360  pp. 
DILLMAN,  A.  C.,  1916.     Breeding  millet  and  sorgo  for  drought  adaptation. 

U.  S.  Dept.  of  Agr.,  Bull.  291,  19  ppr 
DON,  GEORGE,  1838.     A  general    history   of    the     dichlamydeous  plant. 

Vol.  IV.     Corolliflorae. 

DORSET,  M.  J.,  1914.    Pollen  development  in  the  grape  with  special  refer- 
ence to  sterility.     Minnesota  Agr.  Exp.  Sta.,  Bull.  144:  3-60. 

1916.     The  inheritance    and    permanence  of  clonal  varieties.     Proc. 

Amer.  Soc.  Hort.  Sci.,  for  1916:  1-31. 

1919.     A  study  of  sterility  in  the  plum.     Genetics,  4 :  417-488. 
DURST,  C.  E.,  1918.     Tomato  selection  for  fusarium  resistance.     Phyto- 

path.,  8 :  80. 
EAST,  E.  M.,  1907.     The  relation  of  certain  biological  principles  to  plant 

breeding.     Connecticut  Agr.  Exp.  Sta.,  Bull.  158,  92  pp. 

1908a.     Some  essential  points  in  potato  breeding.     Conn.  Agr.  Exp. 

Sta.  Ann.  Report  for  1907-08 :  429-447. 

19086.     A  study  of  the  factors  influencing  the  improvement  of  the 

potatd.     Illinois  Agr.  Exp.  Sta.,  Bull.  127 :  375-456. 

1908c.     Inbreeding  in   corn.     Conn.    Agr.    Exp.    Sta.  Rept.  for  1907 : 

419-428. 

1910a.     The  transmission  of  variations  in  the  potato  in  asexual  repro- 
duction.    Conn.  Agr.  Exp.  Sta.  Rept.  for  1909-10:  119-160. 

19106.     Inheritance  in  potatoes.     Am.  Nat.,  44:  424-430. 

1910c.     The    role    of    hybridization    in    plant  breeding.        Popular 

Science  Monthly,  Oct.,  1910 :  342-355. 

1910d.     Note  on  an  experiment  concerning  the  nature  of  unit  characters. 

Science.  N.  S.  32:  93-95. 

1912a.     A  study  of  hybrids  between  Nicotiana  bigelovii  and  N.  quad- 

rivalvis.     Bot.  Gaz.,  63 :  243-248. 

19126.     Inheritance   of  color  in  the  aleurone  cells  of  maize.     Am. 

Nat.,  46:363-365. 

1916a.     Studies  on  size  inheritance  in  Nicotiana.     Genetics,!:  164-176. 

19166.     Inheritance    in    crosses   between    Nicotiana   langsdorflSi  and 

Nicotiana  alata.    Genetics,  1:  311-333. 

1919a.     Studies  on  self -sterility  III.     The  relation  between  self-fertile 

and  self-sterile  plants.     Genetics,  4 :  340-345. 


LITERATURE  CITATIONS  303 

19196.     Studies  on  self-sterility  IV.     Selective  fertilization.     Genetics, 

4:346-355. 

1919c.     Studies  on  self-sterility   V.     A  family  of  self-sterile  plants 

wholly  cross-sterile  inter  se.     Genetics,  4 :  356-363. 
EAST,  E.  M.  and  HAYES,  H.  K.,  1911.     Inheritance  in  maize.     Conn.  Agr. 

Exp.  Sta.,  Bull.  167 :  142  pp. 

1912.     Heterozygosis  in  evolution  and  in  plant  breeding.     U.  S.  Dept. 

Agr.,  Bur.  Plant  Indust.,  Bull.  243,  58  pp. 
EAST,   E.  M.    and    JONES,    D.  F.,   1919.     Inbreeding    and    outbreeding. 

J.  B.  Lippincott  Co.,  Philadelphia,  285  pp. 
EAST,  E.    M.  and    PARK,    J.  B.,  1917.     Studies   on   self-sterility  I.     The 

behavior  of  self-sterile  plants.     Genetics,  2 :  505-609. 

1918.     Studies  on  self-sterility  II.     Pollen  tube  growth.     Genetics,  3: 

353-366. 
EDGERTON,    C.    W.,    1918.     A  study   of   wilt-resistance   in  the   seed-bed. 

Phytopath.,  8 :  5-14. 
EMERSON,   R.   A.,    1910.     The  inheritance   of  sizes  and  shapes  in  plants. 

Am.  Nat.,  44:739-746. 

1911.     Genetic    correlation    and    spurious    allelomorphism    in    maize. 

Ann.  Kept.  Nebraska  Agr.  Exp.  Sta.,  24:  59-90. 

1912a.     The  unexpected  occurrence  of  aleurone  colors  in  F\  of  a  cross 

between  non-colored  varieties  of  maize.     Am.  Nat.,  46:  612—615. 

19126.     The  inheritance   of  the  ligule  and    auricles  of  corn  leaves. 

Ann.  Rpt.  of  Nebraska  Exp.  Sta.,  25:  81-88. 

1914a.     The  inheritance  of  recurring  somatic  variation  in  variegated  ears 

of  maize.     Neb.  Agr.  Exp.  Sta.  Res.,  Bull.  4:  35  pp. 

19146.     Multiple    factors   vs.     "Golden    Mean"    in  size  inheritance. 

In  Science,  N.  S.,  40 :  57-58. 

1916.  A  genetic  study  of  plant  height  in  Phaseolus  vulgaris.     Neb. 
Agr.  Exp.  Sta.  Res.,  Bull.  7 :  73  pp. 

1917.  Genetical  studies  of  variegated  pericarp  in  maize.     Genetics, 
2:  1-35. 

1918.  A  fifth  pair  of  factors,  Aa,  for  aleurone  color  in  maize,  and  its 
relation  to  the  Cc    and    Rr   pairs.      N.  Y.  Cornell  Univ.   Exp.    Sta. 
Memoir,  16:231-289. 

EMERSON,    R.   A.    and  EAST,   E.    M.,  1913.     The    inheritance  of  certain 

quantitative  characters  in  maize.     Neb.    Agr.    Exp.  Sta.    Res.,  Bull. 

3:  120  pp. 
ENGLEDOW,  F.  L.,  1914.     A  case  of  repulsion  in  wheat.     Proc.  Cambridge 

Phil.  Soc.,  17  :  433-435. 
ETHERIDGE,  W.  C.,  1916.     A  classification  of  the  varieties  of  cultivated  oats. 

New  York  Cornell  Univ.  Exp.  Sta..  Memoir,  10 :  167  pp. 
FARRER,  W.,  1898.     The  making  and  improving  of  wheats  for  Australian 

conditions.     Agr.  Gaz.  of  N.  S.,  Wales,  10:  131-168. 
FLETCHER,   S.  W.,   1911.     Pollination  of  the  Bartlett  and  Kieffer  pears. 

Virginia  Agr.  Exp.  Sta.  Rept.  for  190&-10 :  213-224. 

1916.     North   American  varieties  of  the  strawberry.     Virginia  Agr. 

Exp.  Sta.,  Tech.  Bull.  11:  3-126. 
FOCKE,     W.    O.,     1881.     Die    Pflanzen-mischlinge.     Borntrseger,     Berlin, 

569  pp. 


304  BREEDING  CROP  PLANTS 

FRANDSEN,  H.  N.,  1917.  Die  Befruchtungsverhaltnisse  bei  Grass  und  Klee 
in  ihrer  Beziehung  zur  Zuchtung.  Zeitschr.  fur  Pflanzenzucht.,  5. 

FRASER,  A.  C.,  1919.  The  inheritance  of  the  weak  awn  in  certain  Avena 
crosses  and  its  relation  to  other  characters  of  the  oat  grain.  Cornell 
Univ.  Agr.  Exp.  Sta.  Memoir,  23 :  635-676. 

FREAR,  D.  W.,  1915.  Crossing  of  wheat  flowers  unprotected  after  emascula- 
tion. Jour.  Heredity,  6 :  350. 

FREEMAN,  G.  F.,  1917.  Linked  quantitative  characters  in  wheat  crosses. 
Am.  Nat.,  51 :  683-689. 

1918.  Producing  breadmaking  wheats  for  warm  climates.     Jour.  Here- 
dity, 9:  211-226. 

1919.  Heredity    of  quantitative  characters  in    wheat.     Genetics,   4 : 
1-93. 

FRUWIRTH,  CARL,  1909.  Die  Zuchtung  der  landwirtschaftlichen  Kultur- 
pflanzen.  Paul  Parey,  Berlin.  Bd.  II,  2  Auflage.,  Maize,  pp.  5-40. 
Bd.  Ill,  2  Auflage.,  Flax,  pp.  47-60.  Bd.  IV,  2  Auflage.,  Wheat,  pp. 
107-186;  Barley,  pp.  240-321;  Oats,  pp.  324-365;  Rye,  pp.  187-239; 
Bd.  V,  2  Auflage.,  Rice,  pp.  36-54. 

1912.    Zur  Zuchtung  der  Kartoffel,  Deut.  Landw,  Presse.  39:  551-552, 
565-567. 

1917.  Selection  in  pure  lines.     Jour.  Heredity,  8 :  90-94. 

GAINES,  E.  F.,  1917.  Inheritance  in  wheat,  barley  and  oat  hybrids.  Wash- 
ington Agr.  Exp.  Sta.,  Bull.  135:  3-61. 

1918.  Comparative  smut  resistance  of  Washington  wheats. 
Jour.  Amer.  Soc.  Agron.,  10:  218-222. 

1920.  The  inheritance  of  resistance  to  bunt  or  stinking  smut  of  wheat. 
Jour.  Amer.  Soc.  Agron.,  12:  124-132. 

GALLOWAY,  B.  T.,  1907.     Progress  in  some  of  the  new  work  of  the  Bureau  of 

Plant  Industry.     Yearbook  of  U.  S.  Dept.  of  Agr.  for  1907:  139-141. 
GARBER,  R.  J.,  1921.     A    Preliminary  Note  on  the  Inheritance  of  Rust 

Resistance  in  Oats.     Jour.  Amer.  Soc.  Agron.,  13:  41-43. 
GARBER,  R.  J.,  and  OLSON,  P.  J.,  1919.     A  study  of  the  relation  of  some 

morphological  characters  to  lodging  in  cereals.     Jour.  Am.  Soc.  Agron., 

11:173-186. 
GARNER,    W.    W.,    1912.     Some  observations  on  tobacco  breeding.     Am. 

Breeders'  Assoc.  Rept.,  8 :  458-468. 
GARNER,  W.  W.,  and  ALLARD,  H.  A.,  1920.     Effect  of  the  relative  length  of 

day  and  night  and  other  factors  of  the  environment  on  growth   and 

reproduction  in  plants.     Jour.  Agric.  Research,  18 :  553-606. 
GARTNER,  G.  F.,  1849.     Versuche  und  Beobachtungen  uber  die  Bastarder- 

zeugung  im  Pflanzenreich.  Stuttgart,  Hering  &  Comp.,  791  pp. 
GATES,  R.  R.,  1911.  Mutation  in  Oenothera.  Am.  Nat.,  45:  577-606. 
GILBERT,  A.  W.,  1917.  The  potato.  The  Macmillan  Co.,  New  York, 

318  pp. 
GOFF,  E.  S.,  1894.     Flowering  and  fertilization  of  the  native  plum.     Gard. 

and  For.,  7 :  262-263. 

1901.     Native  plums.     Wis.  Agr.  Exp.  Sta.,  Bull.  87,  31  pp. 
GOODSPEED,  T.  H.,  1912.     Quantitative  studies  of  inheritance  in  Nicotiana 

hybrids.     I.  Univ.  Calif.  Pub.  on  Botany  5 :  87-168. 


LITERATURE  CITATIONS  305 

1913.  Quantitative    studies    of    inheritance    in    Nicotiana    hybrids. 
II.  Univ.  Calif.  Pub.  on 'Botany,  6: 169-188. 

1915.     Parthenocarpy  and  parthenogenesis  in  Nicotiana.     Proc.  Nat. 

Acad.  ScL,  1:341-346. 
GOODSPEED,  T.  H.  and  CLAUSEN,  R.  E.,  1917.     Mendelian  factor  differences 

versus  reaction  system  contrasts  in  heredity.     Am.  Nat.,  51 :  31-46, 

92-101. 
GRAHAM,  R.  J.  D.,   1916.     Pollination  and  cross-fertilization  in  theJuar 

plant    (Andropogon    Sorghum,  Brot.).     Mem.  Dept.  Agr.  India  Bot. 

Ser.,  8:201-216. 
GROTH,  B.  H.  A.,  1910.     Structure  of  tomato  skins.     New  Jersey  Agr.  Exp. 

Sta.,  Bull.  228,  20  pp. 

1911.  The  Fi  heredity  of  size,  shape,  and  number  in  tomato  leaves. 
Parts  1  and  2.     New  Jersey  Agr.  Exp.  Sta.,  Bull.  238  and  239. 

1912.  The  Fi  heredity  of  size,  shape,  and  number  in  tomato  fruits. 
New  Jersey  Agr.  Exp.  Sta.,  Bull.  242. 

1914.  The  "Golden  Mean"  in  the  inheritance  of  size.     In  Science, 
N.  S.,  39:581-584. 

1915.  Some  results  in  size  inheritance.     New  Jersey  Agr.  Exp.  Sta., 
Bull.  278. 

GUIGNARD,   M.  L.,   1899.     Sur  les  antherozoides  et  la  double  copulation 

sexuelle  chez  les  vegetaux  angiospermes.     Rev.   gener. ,  d.    Bot.,    11 : 

129-135. 

1901.     La  double  fecondation  dans  le  mais.     Jour,  de  Bot.,  15 :  37-50. 
HAGEDOORN,  A.  L.,  and  A.  C.,  1914.     Studies  on  variation  and  selection. 

Zeitschr.  fur  Induk.  Abst.  u.  Vererb.,  11:  145-183. 
HANSEN,  N.  E.,  1911.     Some  new  fruits.     South  Dakota  Agr.  Exp.  Sta., 

Bull.  130:  163-200. 

1915.     Progress  in  plant  breeding.     South   Dakota  Agr.   Exp.    Sta., 

Bull.  159:  179-192. 
HARLAN,  H.   V.,   1918.     The  identification  of  varieties  of  barley.     U.  S. 

Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  622,  32  pp. 

1920  Smooth  Awned  barleys,  Jour.  Amer.  Soc.  Agron.  12:  205-208. 
HARLAN,  H.  V.,  and  ANTHONY,  S.  B.,  1920.     Development  of  barley  kernels 

in  normal  and  clipped  spikes  and  the  limitations  of  awnless  and  hooded 

varieties.     Jour.  Agric.  Res.,  19 :  431-472. 

HARLAN,  H.  V.  and  HAYES,  H.  K,  1920.     The  occurrence  of  the  fixed  in- 
termedium Hordeum  intermedium  haxtoni  in  crosses  between  H.  vul- 

gare  pallidum  and  H.  distichon.     Jour.  Agr.  Res.,  19:  575-591. 
HARLAND,  S.  C.,  1917.     On  the  inheritance  of  the  number  of  teeth  in  the 

bracts  of  Gossypium.     West  Indian  Bull.,  16 : 111-120. 

1919a.     Notes  on  inheritance  in  the  cowpea.     Agr.  News.  Barbados, 

18 :  20. 

19196.     Notes  on  inheritance  in  the  cowpea.     Agr.  News.  Barbadose 

18 :  68. 

1919c.     Inheritance  of  certain  characters  in  the  cowpea  (Vigna  sinne- 

sis).     Jour.  Genetics,  8:  101-132. 

1920.    Inheritance  of  certain  characters  in  the  cowpea.    (Vigna  sinnsis.), 

II.    Jour.  Genetics-,  10:  193-205. 
20 


306  BREEDING  CROP  PLANTS 

HARRIS,  J.  A.,  1915.  On  a  criterion  of  substratum  homogeneity  (or  hetero- 
geneity) in  field  experiments.  Am.  Nat.,  49 :  430-454. 

HARSHBERGER,  J.  W.,  1897.  Maize:  A  botanical  and  economic  study. 
In  Contritubions  from  the  Botanical  Laboratory,  Univ.  of  Pennsyl- 
vania, 1 :  75-202. 

1904.  A  study  of  the  fertile  hybrids  produced  by  crossing  teosinte  and 
maize.  In  Contributions  from  the  Botanical  Laboratory,  Univ.  of 
Pennsylvania,  2 :  231-235. 

1909.  Maize,  or  Indian  corn.  Cyclopedia  of  American  agriculture, 
2:398-402. 

HARTLEY,  C.  P.;  BROWN,  ERNEST  B.;  KYLE,  C.  H.;  and  ZOOK,  L.  L.,  1912. 
Crossbreeding  corn.  U.S.  Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  218: 5-72. 

HASSELBRING,  H.,  1912.  Types  of  Cuban  tobacco.  Bot.  Gaz.,  63:  113- 
126. 

HAYES,  H.  K.,  1912.  Correlation  and  inheritance  in  Nicotiana  tabacum. 
Conn.  Agr.  Exp.  Sta.,  Bull.  171,  45  pp. 

1913a.  Corn  improvement  in  Connecticut.  Conn.  Agr.  Exp.  Sta. 
Kept,  for  1913 :  Part  VI,  353-384. 

19136.  The  inheritance  of  certain  quantitative  characters  in  tobacco. 
Zeitsuhr.  fur  Induk.  Abstamm.  u.  Vererb.,  10: 115-129 

1914.  Variation  in  tobacco.     Jour.  Heredity,  5 :  40-46. 

1915.  Tobacco  mutations.     Jour.  Heredity,  6 :  73-78. 

1917.     Inheritance    of    a   mosaic   pericarp    pattern   color   of   maize. 

Genetics,  2:  261-281. 

1918a.     Natural  cross-pollination  in  wheat.     Jour.  Am.   Soc.  Agron., 

10 :  120-122. 

19186.     Normal   self-fertilization   in   corn.     Jour.    Am.    Soc.    Agron., 

10:  123-126 

1918c.     Natural  crossing  in  wheat.     Jour.   Heredity,    9:  326-330,, 
HAYES,   H.   K.  and  ARNY,   A.   C.,    1917.     Experiments  in  field  technic  in 

rod-row  tests.     Jour.  Agr.  Res.,  11 :  399-419. 
HAYES,  H.  K.  and  BEINHART,  E.  G.,  1914.     Mutation  in  tobacco.     Science 

tf.  S.  39:  34-35. 
HAYES,  H.  K.  EAST,  E.  M.  and  BEINHART,  E.  G.,  1913.     Tobacco  breeding 

in  Connecticut.      Conn.  Agr.  Exp.  Sta.,  Bull.  176. 
HAYES,  H.  K.  and  EAST,  E.  M.,  1911.     Improvement  in  corn.     Conn.  Agr. 

Exp.  Sta.,  Bull.  168:  3-21. 

1915.     Further  experiments  on  inheritance  in  maize.     Conn.  Agr.  Exp. 

Sta.,  Bull.  188,  31  pp. 

HAYES,  H.  K.  and  GARBER,  R.  J.,  1919.  Breeding  small  grains  in  Minne- 
sota. Part  1.  Technic  and  results  with  wheat  and  oats.  Minnesota 

Agr.  Exp.  Sta.,  Bull.  182. 

1919.     Synthetic   production  of  high-protein  in  corn   in  relation  to 

breeding.     Jour.  Am.  Soc.  Agron.,  11:  309-318. 
HAYES,  H.  K.  and  HARLAN,  H.  V.,  1920.     The  inheritance  of  the  length  of 

internode  in  the  rachis  of  the  barley  spike.     U.  S.  Dept.  Agr.   Bull. 

869,  26  pp. 

HAYES,  H.  K.  and  JONES,  D.  F.,  1916.  The  effects  of  cross-  and  self-fer- 
tilization in  tomatoes.  Conn.  Agr.  Exp.  Sta.  Ann.  Rept.  for  1916: 

305-318. 


LITERATURE  CITATIONS  307 

HAYES,  H.  K.  and  JONES,  D.  F.,     1916.     First  generation  crosses  in  cu- 
cumbers.    Conn.  Agr.  Exp.  Sta.  Ann.  Kept,  for  1916:  319-322. 
HAYES,   H.    K.  and  OLSON,  P.  J.,  1919.     First  generation  crosses  between 

standard  Minnesota  corn  varieties.     Minnesota  Agr.  Exp.  Sta.,  Bull. 

183:  4-22. 
HAYES,  H.  K.  PARKER,  J.  H.,  and  KURTZWEIL,  CARL,  1920.     Genetics  of 

rust  resistance  in  crosses  of  varieties  of  Triticum  vulgare  with  varieties 

of  T.  durum  and  T.  dicoccum.     Jour.  Agr.  Res.,  11 :  523-542. 
HAYES,  H.  K.  and  STAKMAN,  E.  C.,  1919.     Rust  resistance  in  timothy. 

Jour.  Am.  Soc.  Agron.,  11 :  67-70. 
HAYS,  W.   M.,  and  Boss,  ANDREW,   1899.     Wheat.     Varieties,  breeding, 

cultivation.     Minnesota  Agr.  Exp.  Sta.,  Bull.  62:  321-494. 
HAYS,  W.  M.,  1901.     Plant  breeding.     U.  S.  Dept.  Agr.,  Div.  Veg.  Phys. 

and  Path.  Bull.  29,  72  pp. 
HECKEL,  E.,  1909.     Fixation  de  la  mutation  germinaire  culturale  du  Sola- 

num  maglia:  variation  de  forme  et  de  coloris  des  tubercules  unites. 

Compt.  Rend.  1'  Acad.  Sci.,  149 :  831-833. 

1912.     Sur  l,a  mutation  germinaire  culturale  du  Solanum  tuberosum  L. 

Compt.  Rend.  1'  Acad.  Sci.,  155 :  469-471. 
HECTOR,  G.  P.,  1913.     Notes  on  pollination  and  cross-fertilization  in  the 

common  rice  plant,  Oryza  sativa,    Linn.     Mem.    Dept.    Agr.    India, 

Bot.  Ser.,  6:  1-10. 

1916.     Observation  on  the  inheritance  of  anthocyan  pigment  in  paddy 

varieties.     Mem.  Dept.  Agr.  India,  Bot.  Ser.,  8:  89-101. 
HEDRICK,  U.  P.,  and  BOOTH,  N.  O.,  1907.     Mendelian  characters  in  toma- 
toes.    Proc.  Soc.  for  Hort.  Sci.,  5:  19-24. 

HEDRICK,  U.  P.  and  ANTHONY,  R.  D.,  1915.     Inheritance  of  certain 'charac- 
ters of  grapes.   N.  Y.  (Geneva)  Agr.  Exp.  Sta.,  Tech.  Bull.  45:  3-  9. 
HEDRICK,  U.  P.  and  WELLINGTON,  R.,  1912.     An  experiment  in  breeding 

apples.     N.  Y.  (Geneva)  Agr.  Exp.  Sta.,  Bull.  350:  141-186. 
HENKEMEYER,  A.,  1915.     Untersuchungen  iiber  die  Spaltungen  von  Weizen- 

bastarden  in  der  F2  und  F3  Generation.     Jour,  f  iir  Land.,  63 :  97-124. 
HILSON,  G.  R.,  1916.     A  note  on  the  inheritance  of  certain  stem  characters 

in  sorghum.     Agr.  Jour,  of  India,  11 :  150-155. 
HOPKINS,  C.  G.,  1899.     Improvement  in  the  chemical  composition  of  the 

corn  kernel.     Illinois  Agr.  Exp.  Sta.,  Bull.  55:  205-240. 
HOSHINO,  Y.,  1915.     On  the  inheritance  of  the  flowering  time  in  peas  and 

rice.     Jour.  Col.  Agr.  Tokoku  U.  (Sapporo),  6 :  229-288. 
HOUSER,    TRUE,    1912.     Certain   results  in  Ohio  tobacco  breeding.     Am. 

Breeders'  Assoc.  Rept.,  8 :  468-479. 
HOWARD,   A.;   HOWARD,   G.;  and   RAHMAN  KAHN,   ABDUL,   1910a.     The 

economic  significance  of  natural  cross-fertilization  in  India.     Mem. 

Dept.  Agr.  India,  Bot.  Ser.,  3 : 

1919.     Studies   of   the   pollination   of   crops.     I.  Mem.    Dept.    Agr. 

India,  Bot.  Ser.,  10:  195-220. 
HOWARD,   A;  and  HOWARD,  G.  L.  C.,  19106.     Studies  in  Indian  tobaccos, 

No.  1.    The  types  of  Nicotiana  rustica  L.     Yellow-flowered  tobacco. 

Mem.  Dept.  Agr.  India,  Bot.  Ser.,  3 :  1-58. 

1910c.     Studies  in  Indian  tobaccos,  No.  2.     The  types  of  Nicotiana 

tabacum  L.     Mem.  Dept.  Agr.  India,  Bot.  Ser.,  3 :  59-176. 


308  BREEDING  CROP  PLANTS 

1912.  On  the  inheritance  of  some  characters  in  wheat.     Mem.  Dept. 
Agr.  India,  Bot.  Ser.,  6 :  1-47. 

1915.     On  the  inheritance  of  some  characters  in  wheat.     II.  Mem. 
Dept.  Agr.  India,  Bot.  Ser.,  7 :  273-285. 

HOWARD,  G.  L.  C.,  1913.  Studies  in  Indian  tobaccos,  No.  3.  The  inherit- 
ance of  characters  in  Nicotiana  tabacum  L.  Mem.  Dept.  Agr.  India, 
Bot.  Ser.,  6:25-114. 

HUTCHESON,  T.  B.,  1914.  Thirteen  years  of  wheat  selection.  Am.  Nat., 
48:459-466. 

HUTCHESON,  T.  B.  and  WOLFE,  T.  K.,  1917.  The  effect  of  hybridization 
on  maturity  and  yield  in  corn.  Virginia  Agr.  Exp.  Sta.,  Tech.  Bull. 
18:  161-170. 

IKENO,  S.,  1913.     Studien  uber  die  Bastarde  von  Paprika  (Capsicum  an- 
nuum).     Zeitschr.  fiir  Induk.  Abstamm.  u.  Vererb.,  10:  9&-114. 
1914.     tTber  die  Bestaubung  und  die  Bastardierung  von  Reis.     Zeit- 
schr. fiir  Pflanzenzucht. ,    2:  495-503. 

1918.  "  Zikken-Idengaku  "  (a  text-book  on  genetics)  (Japanese).     3rd 
edition.     Nippon-no  Romazi-Sya,  Tokyo.     Abstract  in  Bot.  Abstracts, 
2:  114,  115. 

JARDINE,  W.  M.,  1917.  A  new  wheat  for  Kansas.  Jour.  Am.  Soc.  Agron., 
9:257-266. 

JELINEK,  J.,  1918.  Beitrag  zur  Technik  der  Weizenbastardierung.  Zeit- 
schr. fiir  Pflanzenzucht.,  6:  55-57 

JENKINS,  E.  H.,  1914.  Studies  on  the  tobacco  crop  of  Connecticut.  Conn. 
Agr.  Exp.  Sta.,  Bull.  180,  65  pp. 

JENSEN,  H.,  1907.  Tobacco  experiments.  Jaarb.  Dept.  Landb.  Nederland. 
Indie,  1907 :  199-217.  Abstract  in  Exp.  Sta.  Record,  20 :  935. 

JESENKO,  F.,  1911  (1913).  Sur  un  hybride  fertile  entre  Triticum  sativum  9 
(Ble  Mold  Squarehead)  et  Secale  cereale  o*  (Seigle  de  Petkus). 
Int.  Conf.  de  Genetique,  4:  501-511. 

1913.  tlber    Getreide — Speziesbastarde    (Weizen-Roggen).     Zeitschr. 
fiir  Induk.  Abstamm.  u.  Vererb.,  10:  311-326. 

JOHANNSEN,  W.,  1903.     Ucber  Erblichkeit  in  Populationen  und  in  reinen 

Linien.     G.  Fischer,  Jena.,  68  pp. 

1909.     Elemente  der    exakten    Erblichkeitslehre.     G.  Fischer,  Jena., 

723  pp. 

JOHNSON,  D.  S.,  1915.     Sexuality  in  plants.     Jour.  Heredity,  6 :  3-16. 
JOHNSON,    J.,    1919.     An  improved  strain  of  Wisconsin  tobacco.     Jour. 

Heredity,  10:281-288. 

1919.  Inheritance  of  branching  habit  in  tobacco.     Genetics,  4 :  307- 
340. 

JONES,  D.  F.,  1916.  Natural  cross  pollination  in  the  tomato.  Science  N.  S., 
43 :  509-510. 

1917.  Linkage  in  Lycopersicum.     Am.  Nat.,  61:  608-621. 

1918.  The  effects  of  inbreeding  and  cross-breeding  upon  development. 
Conn.  Agr.  Exp.  Sta.,  Bull.  207,  100  pp. 

1919.  Selection   of  pseudo-starchy  endosperm  in  maize.     Genetics, 
4:  364-393. 

1920.  Selection  in  self-fertilized  lines  as  a  basis  for  corn  improvement, 
Jour.  Amer.  Soc.  Agron.,  12:  77-100. 


LITERATURE  CITATIONS  309 

JONES,    D.    F.,    1920.     Heritable   characters   of    maize — Defective   seeds. 

Jour.  Heredity,  11:  161-167. 
JONES,  D.  F.  and  GALLASTEGUI,  G.  A.,   1919.     Some  factor  relations  in 

maize  with  reference  to  linkage.     Am.  Nat.,  53 :  239—246. 
JONES,  D.  F.,  HAYES,  H.  K.,  SLATE,  JR.,  W.  L.,  and  SOUTHWICK,  B.  G., 

1917.  Increasing  the  yield  of  corn  by  crossing.      Conn.  Agr.  Exp.  Sta. 
Kept,  for  1916  :  327-347. 

JONES,  L.  R.,  and  GILMAN,  J.  C.,  1915.  The  control  of  cabbage  yellows 
through  disease  resistance.  Wis.  Agr.  Exp.  Sta.,  Res.  Bull.  38,  70  pp. 

JONES,  L.  R.,  WALKER,  J.  C.,  and  TISDALE,  W.  B.,  1920.  Fusarium  Resis- 
tant Cabbage.  Wisconsin  Agr.  Exp.  Sta.,  Res.  Bull.  48:  3-34. 

KAJANUS,  B.,  1913.  Uber  die  Vererbungsweise  gewisser  Merkmale  der 
Beta-  und  Brassica-Riiben.  Zeitschr.  fur  Pflanzenziicht.,  1:  125 — 186. 

KARPER,  R.  E.,  and  CONNER,  A.  B.,  1919.  Natural  cross-pollination  in  milo. 
Jour.  Am.  Soc.  Agron.,  11 :  257-259. 

KEARNEY,  T.  H.,  1914.  Mutation  in  Egyptian  cotton.  Jour.  Agr.  Res., 
2 :  287-302. 

KEEBLE,  F.,  and  PELLEW,  C.,  1910.  The  mode  of  inheritance  of  stature  and 
of  time  of  flowering  in  peas  (Pisum  sativum).  Jour.  Genetics,  1: 
47-56. 

KEZER,  ALVIN  and  BOYACK,  BREEZE,  1918.  Mendelian  inheritance  in  wheat 
and  barley  crosses.  Colorado  Agr,  Exp.  Sta.,  Bull.  249,  139  pp. 

KIESSELBACH,  T.  A.,  1916.  Recent  developments  in  our  knowledge  con- 
cerning corn.  Nebraska  Corn  Improv.  Assoc.  Rept.,  7  :  15-42. 

1918.  Studies  concerning  the   elimination  of  experimental  error  in 
comparative  crop  tests.     Nebraska  Agr.  Exp.  Sta.,  Res.  Bull.  13,  95  pp. 

KIESSELBACH,  T.  A.  and  RATCLIFF,  J.  A.,  1917.  Oats  investigations.  Ne- 
braska Agr.  Exp.  Sta.,  Bull.  160.  48  pp. 

KIRCHNER,  O.,  1886.  Flora  von  Stuttgart  und  Umgebung,  mit  besonderer 
Beriicksichtigung  der  Pflanzenbiologischen  Verhaltnisse.,  Stuttgart. 

KNIGHT,  L.  I.,  1917.  Physiological  aspects  of  self-sterility  of  the  apple. 
Proc.  Am.  Soc.  Hort.  Sci.,  1917 :  101-105. 

KNIGHT,  T.  A.,  1841.  Philisophical  transactions.  See  "Knight's  Col- 
lected Works,"  London  (1799). 

KNUTH,  PAUL,  1909.  Handbook'  of  flower  pollination.  Translation  by 
Davis.  Clarendon  Press.  Vol.  II,  Flax,  p.  216.  Vol.  Ill,  Maize, 
p.  518,  wheat,  p.  529,  oats,  p.  524,  barley,  p.  532,  rye,  p.  531,  rice, 
p.  521. 

KOCK,  L.,  1917.  Experiments  on  the  artificial  hybridization  of  rice  in 
Java.  Int.  Rev.  Sci.  and  Prac.  Agr.,  8 :  1240-1241. 

KOLREUTER,  J.  G.,  1766.  Dritte  Fortsetzung  der  vorlaufigen  Nachricht 
von  einigen  das  Geschlecht  der  Pflanzen  betreffenden  Versuchen  und 
Beobachtungen.  Gleditschen  Handlung.  Reprinted  1893  in  Ost- 
wald's  Klassiker  der  exakten  Wissenschaften  No.  41.  Leipzig:  Eng- 
elmann. 

KUWADA,  Y.,  1919.  Die  Chromosomenzahl  von  Zea  mays,  L.  Ein  Beitrag 
zur  Hypothesa  der  Individuality  der  Chromosomen  und  zur  Frage 
uber  die  Herkunft  von  Zea  mays,  L.  Jour.  Coll.  Sci.  Imperial  Univ. 
Tokyo,  39:  1-148.  Abst.  by  Ikeno,  S.  in  Bot.  Absts.,  4:  104,  1920. 


310  BREEDING  CROP  PLANTS 

LEAKE  H.  M.,  1911.     Studies  in  Indian  cotton.     Jour.  Genetics,  1 :  205-272. 
LEIDIGH,  A.  H.,  1911.     Methods  for  the  improvement  of  sorghum.     Am. 

Breeders'  Mag.,  2  :  294-295. 
LEIGHTY,    C.    E.,    1915.     Natural    wheat-rye    hybrids.     Jour.    Am.   Soc. 

Agron.,  7 :  209-216. 

1916.     Carman's  wheat-rye  hybrids.     Jour.  Heredity,  7  :  420-427. 
LEIGHTY,  C.  E.,  and  HUTCHESON,  T.  B.,  1919.     On  the  blooming  and  fer- 
tilization of  wheat  flowers.     Jour.  Am.  Soc.  Agron.,  11 :  143-162. 
LINDSTROM,   E.  W.,  1918.     Chlorophyll  inheritance  in  maize.     New  York 

Cornell  Agr.  Exp.  Sta.,  Mem.  13:  3-68. 
LEWIS,  C.  I.,  and  VINCENT,  C.  C.,  1909.     Pollination  of  the  apple.     Oregon 

Agr.  Exp.  Sta.,  Bull.  104,  3-40. 
LOTSY,  J.  P.,  1916.     Evolution  by  means  of  hybridization.     The  Hague, 

Martinus  Nykoff,  pp.  68-75. 
LOVE,  H.  H.,  1914.     Oats  for  New  York.     N.  Y.  Cornell  Agr.  Exp.  Sta., 

Bull.  343:  362-416. 
LOVE,  H.  H.  and  CRAIG,  W.  T.  1918a.     Methods  used  and  results  obtained 

in  cereal  investigations  at  the  Cornell  station.     Jour.  Am.  Soc.  Agron., 

10:  145-157. 

19186.     Small  grain  investigations.     Jour.  Heredity,  9:  67-76. 

1918c.     The  relation  between  color  and  other  characters  in  certain 

Avena  crosses.     Am.  Nat.,  52:  369-383. 

1919a.     Fertile  wheat-rye  hybrids      Jour.  Heredity,  10:  195-207. 

19196.     The  synthetic  production  of  wild  wheat  forms.     Jour.   He- 
redity, 10:51-64. 
LOVE,  H.  H.  and  McRosTiE,  G.  P.  1919.     The  inheritance  of  hull-lessness 

in  oat  hybrids.     Am.  Nat.,  53 :  5-32. 
LUMSDEN,  D.,  1914.     Mendeliem  in  melons.     New  Hampshire  Agr.  Exp. 

Sta.,  Bull.  172;  58  pp. 
MCCLELLAND,  C.  K,  1919.     The  velvet  bean.     Georgia  Agr.  Exp.  Sta., 

Bull.  129:  83-98. 

MCFADDEN,  E.  A.,  1917.     Wheat-rye  hybrids.     Jour.  Heredity,  8:  335-336. 
McLENDON,     C.    A.,    1912.     Mendelian    inheritance   in   cotton   hybrids. 

Georgia  Agr.  Exp.  Sta.,  Bull.  99:  139-228. 
McRosTiE,  G.  P.,  1919.     Inheritance  of  anthracnose  resistance  as  indicated 

by  a  cross  between  a  resistant  and  a  susceptible  bean.     Phytopath.,  9 : 

141-148. 

1921.     Inheritance    of    Disease    Resistance    in    the    Common   Bean. 

Jour.  Amer.  Soc.  Agron.,  13:  15-32. 
MACDOUGAL,  D.  T.,  1916.    -Analysis  of  a  potato  hybrid,  Solanum  fendleri  X 

S.  tuberosum.     Carnegie  Inst.  Yearbook,  16:  98. 
MACOUN,   W.  T.    1915.     Plant  breeding  in  Canada.     Jour.  Heredity,  6: 

398-403. 

1918.     The  potato  in  Canada.     Canadian  Exp.  Farm,  Bull.  90:  3-100. 
MALTE,  M.  O.   and  MACOUN,  W.  T.,  1915.     Growing  field  root,  vegetable 

and  flower  seeds  in  Canada.     Canadian  Dept.  of  Agr.,  Bull.  22,  Second 

series,  15  pp. 
MARTIN,  J.  N.,  1913.     The  physiology  of  the  pollen  of  Trifolium  pratense. 

Bot.  Gaz.,  56:  112-126, 


LITERATURE  CITATIONS  311 

MAYER-GMELIN,  H.,  1917.     De  Kruising  van  roode  onge  baarde  spelt  met 

fluweelkaf  Essex-Tarwe  'een  voorbeeld  van    Factoren-analyze.     Cul- 

tura  29 : 141-159.    See  Abstract  Bulletin  Monthly  Intelligence  and  Plant 

Diseases,  8 : 1236-1239  (1917),  Zeitschr.  fur  Induk.  Abstamm.  u.  Vererb., 

29:  51  (1918). 
MENDEL,  GREGOR  JOHANN,   1909.     Experiments  in    plant   hybridization. 

In  Bateson's  "Mendel's  Principles  of  Heredity,"  pp.  317-368.     1909. 

Translated  by  Royal  Horticultural  Society. 
MERCER,  W.  B.,  and  HALL,  A.  D.,  1911.     The  experimental  error  of  field 

trials.     Jour.  Agr.  Sci.,  4 :  107-132. 
MILLER,   EDWIN   C.,    1919.     Development  of  the  pistillate  spikelet  and 

fertilization  in  Zea  Mays  L.     Jour.  Agr.  Res.,  18:  255-265. 
MIYAZAWA,  B.,  1918.     Oomugi  no  mi  no  iro  iden  ni  tuite  (on  the  inheritance 

of  the  fruit-color  of  barley).     Bot.  Mag.  Tokyo,  32 :  308-310.     Abstract 

in  Bot.  Abstrs.,  1 :  243  Entry.  1537:  1919. 
MONTGOMERY,    E.    G.,    1906.     What   is    an   ear   of  corn?     Popular  Sci. 

Mthly.,  68 :  55-62. 

1909.     Experiments  with  corn.     Nebraska  Agr.  Exp.  Sta.,  Bull.  112: 

5-36. 

1912.  Wheat  breeding  experiments.     Nebraska  Agr.  Exp.  Sta.,  Bull. 
125,  16  pp. 

1913.  Experiments  in  wheat  breeding.     U.  S.  Dept.  Agr.,  Bur.  Plant 
Indust.,Bull.  269,  61  pp. 

MORROW,  G.  E.,  and  GARDNER,  F.  D.,  1893.     Field  experiments  with  corn. 

Illinois  Agr.  Exp.  Sta., Bull.  25:  173-203. 

1894.     Experiments  with  corn.     Illinois  Agr.  Exp.  Sta.,  Bull.  31:  359- 

360. 
MUNSON,  W.  M.,  1906.     Plant  breeding  in  relation  to  American  pomology. 

Maine  Agr.  Exp.  Sta.,  Bull.  132:  149-176. 
NABOURS,   ROBERT  K.,   1919.     Parthenogenesis  and  crossing-over  in  the 

grouse  locust  Apotettix.     Am.  Nat.    63 :  131-142. 
NAUDIN,  CH.,  1856.     Nouvelles  recherches  sur  les  caracters  specifiques  et 

les  varietes  des  plantes  du  genre  Cucubita.     Ann.  d.  Sci.  Nat.     Fourth 

series,  6 :  5-73. 

1859a.     Especes  et  des  varietes  du  genre  Cucumis.     Ann.  d.  Sci.  Nat. 

Fourth  series,  11 :  1-87. 

18596      Revue  des  Cucurbitacees.     Ann.  d.  Sci.  Nat.     Fourth  series, 

12:79-164. 

1865.     Nouvelles     recherches     sur'     1'hybridite     dans    les    vegetaux. 

Nouvelles   archives   du'    Museum   d'    Histoire   Naturelle  de   Paris  I. 

25-176. 
NAWASCHIN,  S.,  1898.     Resultate  einer  Revision  der  Befruchtungsvorgange 

bei  Lilium  martagon  und  Fritillaria  tenella.     Bull.  Acad.  Imp.  Sci.  St. 

Petersbourg.  s.  5,  t.  9,  No.  4:  377-382. 

NEALE,  S.  T.,  1901.     Pedigreed  sorghum  as  a  source  of  cane  sugar.     Dela- 
ware Agr.  Exp.  Sta.,  Bull.  51,  24  pp. 
NEWMAN,  L.  H.,  1912.     Plant  breeding  in  Scandinavia.     Ottawa,  Canada, 

193  pp. 


312  BREEDING  CROP  PLANTS 

NILSSON-EHLE,  H.,  1908.     Einige  Ergebnisse  von  Kreuzungen  bei  Hafer 
und  Weizen.     Botan.  Notiser,  Lund.,  pp.  257-298. 
1909.     Kreuzungsimtersuchungen    an    Hafer    und     Weisen.     Lunds. 
Univ.  Arsskr.  N.  F.  Afd.  2,  Bd.  5  Rr.  2pp.  122. 

191  la.     Ueber  Falle  spontanen   Wegf aliens  eines   Hemmungsfaktors 
beim  Hafer      Zeitzchr.  fiir  Induk.  Abstamm.  u.Vererb.,  5:  1-37. 
19116.     Kreuzungsuntersuchungen    an    Hafer    und    Weizen.     Lunds. 
Univ.  Arsskr.  N.  F.  Afd.2,  Bd.  7,  Nr.  6,  pp.  3-84. 

1911c.  Mendelisme  et  Acclimatation.  Int.  Conf .  de  Genetique,  4 : 1- 
22. 

1912.  Zur   Kenntnis   der   Erblichskeitsverhaltnisse   der   Eigenschaft 
Winterfestigkeit    beim     Weizen.     Zeitschr.     fiir     Pflanzenzucht.,     1: 
3-12. 

1915.  Gibt  es  erbliche  Weizenrassen  mit  mehr  oder  weniger  vollstand- 
iger  Selbstfruchtung?  Zeitschr.  fur  Pflanzenziicht.,  3:  1-6. 

1917.  Improvement  of  black  oats  by  selection  and  crossing  in  Sweden. 
Int.  Rev.  Sci.  and  Prac.  of  Agr.  Year  8,  No.  5:  712-715. 

NILSSON,  H.  N.,  1901.  Forteckning  ofver  de  vigtigarte  sorteng  pa  Sveriges 
utsades  forenings  forsoksfalt.  Sveriges  Utsad.  Tidskr.,  1901,  66-104. 
From  Etheridge,  1916. 

1912-13.  Potatis  foradling  och  potatis  bedomning.  Weibulls 
arsbok,  8:  4-31.  1913.  Abstract  by  author  in  Zeitschr.  fiir  Pflanzen- 
zucht., 1:  240-242,  1912-1913. 

NILSSON,  HERIBERT,  1916.  Populations  analysen  und  Erblichkeitsversuche 
tiber  die  Selbststerelitat,  Selbstfertilitat,  und  Sterilitat  bei  dem  Roggen. 
Zeitschr.  fiir  Pflanzenziicht.,  4:  1-44 

NOHARA,  S.,  1918.  Endo  no  keisitu  iden  ni  .tuite.  (On  the  inheritance  of 
certain  characters  in  the  pea).  (In  Japanese)  Nippon  Ikusyugakukwai 
Kwaiho  (Rep.  Jap.  Assoc.  Breeding  Sci.)  22:  12-14,  1918.  Genetic 
studies  in  some  characters  in  Pisum  Bot.  Mag.  Tokyo  32:  91-102, 

1918.  Abstract  by  S.  Ikeno  in  Bot.  Abstrs.,  1 :  87.     Entry  491,  1918. 
NORTON,    J.   B.,    1902.     Improvement  of   oats   by   breeding.     Proc.    Int. 

Conf.  Plant  Breeding  and  Hybridization,  1:  103-109. 

1907.     Notes  on  breeding  oats.     Proc.  Am.  Breeders,  Assoc.,  3  :280-285. 

1911-12.      Asparagus  breeding.     Am.  Breeders'  Assoc.  Rept.,  7  &  8: 

440-444. 

1913.  Methods  used  in  breeding  asparagus  for  rust  resistance.     U.  S. 
Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  263,  60  pp. 

1919.  Washington  asparagus :  Information  and  suggestions  for  growers 
of  new  pedigreed  rust-resistant  strains.     U.  S.  Dept.  Agr.,  Bur.  Plant 
Indust.,  Cotton,  Truck  and  Forage  Crop  Disease  Invest.  Cir.  7 :  2-8. 

OLIVER,  G.  W.,  1910.     New  Methods  of  plant  breeding.     U.  S.  Dept.  Agr., 

Bur.  Plant  Indust.,  Bull.  167,  39  pp. 
OLSON,  P.  J. ;  BULL,  C.  P.;  and  HAYES,  H.  K.,  1918.     Ear  type  selection 

and  yield  in  corn.     Minnesota  Agr.  Exp.  Sta.,  Bull.  174:  3-60. 
ORTON,   W.  A.,   1902.     The  wilt  disease  of  the  cowpea  and  its  control. 

Part  I.     U.  S.  Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  17:  9-22. 

1911  (1913).     The  development  of  disease  resistant  varieties  of  plants. 

Int.  Conf.  de  Genetique,  4:  247-261. 


LITERATURE  CITATIONS  313 

1914.     Potato  wilt,  leaf  roll,  and  related  diseases.     U.  S.  Dept.  Agr., 

Bull.  64,  48  pp. 
PAMMEL,  L    H.,   1892.     On  the  crossing  of  cucurbits.     Iowa  Agr.  Exp. 

Sta.,  Bull.  19:  595-600. 
PARKER,  JOHN  H.,  1918.     Greenhouse  experiments  on  the  rust  resistance  of 

oat  varieties.     U.  S.  Dept.  Agr.,  Bull.  629,  1-16. 

1920.     A  preliminary  study  of  the  inheritance  of  rust  resistance  in  oats. 

Jour.  Am.  Soc.  Agron.,  12 :  23-38. 
PARKER,  W.  H.,  1914.     Lax  and  dense-eared  wheats.     Jour.  Agr.  Sci.,  6 : 

371-386. 

PARNELL,  F.  R.,  AYYANGAR,  G.  N.  R.,  and  RAMIAH,  K.,  1917.     The  inheri- 
tance of  characters  in  rice.     1.  Mem.  Dept.  Agr.  India,  Bot.  Ser.,  9: 

75-105. 
PEARL,  RAYMOND,  1915.     Modes  of  research  in  genetics.     The  Macmillan 

Co.,  New  York. 
PEARL  R.,  and  MINER,  J.  R.,  1914.     A  table  for  estimating  the  probable 

significance  of  statistical  constants.     Maine  Agr.  Exp.  Sta.,  Bull.  226: 

85-88. 
PIPER,  C.  V.,  1912.     Agricultural  varieties  of  the  cowpea  and  immediately 

related  species.     U.  S.  Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  229:  1-160. 

1916.     Forage      plants     and      their     culture.     The    Macmillan  Co., 

New  York,  618  pp. 
PIPER,  C.  V.  EVANS,   M.  W.,  McKEE,  R.,  and  MORSE,  W.  J.,  191£.     Alfalfa 

seed  production;  pollination  studies.     U.  S.  Dept.  Agr.,  Bull.  75,  32  pp. 
PIPER,  C.  V.  and  MORSE,  W.  J.,  1910.     The  soybean:  History,  varieties  and 

field  studies.     U.  S.  Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  197,  84  pp. 
POWELL,     G.    H.,     1898.     Plant     breeding.     Am.     Gard.,    16:    466-467, 

514-515. 

PRICE,  H.  L.,  and  DRINKARD,  A.  W.,  JR.,  1908.     Inheritance  in  tomato  hy- 
brids.    Virginia  Agr.  Exp.  Sta.,  Bull.  177:  18-53. 

1911-12.      Inheritance    in     cabbage    hybrids.      Ann.    Rept.   Virginia 

Polytec.  Inst.  Agr.  Exp.  Sta.  for  1911  and  1912 :  240-257. 
PRIDHAM,  J.  T.,    1916.     Oat    breeding    experiments.     Agr.     Gaz.    N.  S. 

Wales,  27:457-461. 
QUANJER,  H.  M.,  1920.     The  mosaic  disease  of  the  Solanacese,  its  relation  to 

the  phloem-necrosis  and  its  effect  upon  potato  culture.     Phytopath., 

10 :  35-47. 
REEVES,    F.    S.,  1918.     Report  of    Hort.    Exp.    Sta.,    Vineland,  Ontario, 

for  1916-17:39-41. 
RIOLLE,  Y.  T.,  1914.     Recherches  morphologiques  et  biologiques  sur  les 

Radies  cultives.     Ann.  Sci.  Agron.,   31st  year:  346-550. 
ROBERTS,    HERBERT    F.,    1919.     The   founders    of    the    art    of    breeding. 

Jour.  Heredity,  10:  99-106,  147-152,  229-239,  257-275. 

1919.     The  contribution  of  Carl  Friedrich  von  Gartner  to  the  history  of 

plant  hybridization.     Am.  Nat.,  53 :  431-445. 
RUMKER,    K.   VON,    1913.     Ueber    Roggenzuchtung.     Btr.    zur   Pflanzen- 

ziicht.,  3 :  8-28. 

RUMKER,  K.  VON,  and    LEIDNER,    R.,    1914.     Ein    Beitrag  zur    Frage  der 
Inzucht  bei  Roggen.     Zeitschr.  fur  Pflanzenziicht.,  2 :  427-444. 


314  BREEDING  CROP  PLANTS 

RUSSELL,  H.  L.,  and  MORRISON,  F.  B.,  1919.     Service  to  Wisconsin.     Wis. 

Agr.  Exp.  Sta.,  Bull.  302,  35  pp. 
SAGERET,  A.  1826.     Considerations  sur  la  production  des  hy brides,  des  vari- 

antes  et  des  varietes  en  general,  et  sur  celles  de  la  famille  des  Cucur- 

bitacees  en  particulier.  .  Ann.  d.  Sci.  Nat.,  8:  294-314. 
SALAMAN,  R.  N.,  1910—11.     The  inheritance  of  color  and  other  characters 

in  the  potato.     Jour.  Genetics,  1 :  7-46. 

1912-13.     A  lecture   on    the    hereditary    characters  in   the  potato. 

Jour.  Roy.  Hort.  Soc.,  38:  34-39. 

1909-11.     Male  sterility  in  potatoes,  a  dominant  Mendelian  character; 

with  remarks  on  the  shape  of  the  pollen  in  wild  and  domestic  varieties. 

Jour.  Linnean  Soc.,  39:  301-312. 

1911  (1913).     Studies  in  potato  breeding.     Int.  Conf.  de  Genetique, 

4 :  373-375. 
SALMON,  CECIL,  1914.     Sterile  florets  in  wheat  and  other  cereals.     Jour. 

Am.  Soc.  Agron.,  6:  24-30. 
SAUNDERS,    C.    E.,    1905.     A    natural    hybrid    in    wheat.     Proc.   Amer. 

Breeders'  Assoc.,  1:  137-138. 

1912.     Marquis   wheat.     Exp.    Farms'    Rept.,    Ottawa,  Canada,  pp. 

118-120. 
SCOTT,  W.  B.,  1917.     The    theory     of    evolution.     The   Macmillan    Co., 

New  York,  183  pp. 
SETCHELL,  W.  A.,  1912.     Studies  in    Nicotiana.     1.  Un  v.  Calif.  Pub.  in 

Bot.,  5:  1-86. 
SHAMEL,  A.  D.,  and  COBBY,  W.  W.,  1907.     Tobacco  breeding.     U.  S.  Dept. 

Agr.,  Bur.  Plant  Indust.,  Bull.  96,  71  pp. 

1910.     Tobacco  breeding.     Am.    Breeders'  Assoc.  Rept.,  6:  268-275. 
SHAMEL,  A.    D.,    SCOTT,    L.    B.,    and    POMEROY,     C.    S  ,    1918.     Citrus- 
fruit    improvement:  A    study  of   bud    variation  in  the  Washington 

navel  orange.     U.  S.  Dept.  of  Agr.,  Bull.  623,  146  pp. 

1918.     Citrus-fruit  improvement:  A  study  of  bud  variation  in  the 

Valencia  orange.     U.  S.  Dept.  of  Agr.,  Bull.  624,  120  pp. 

1918.  Citrus-fruit  improvement:  A  study  of  bud  variation  in  the 
Marsh  grapefruit.     U.  S.  Dept.  of  Agr.,  Bull.  697,  112  pp. 

1919.  Origin  of  a  new  and  improved  French  prune  variety.     Jour. 
Heredity,  10:339-343. 

SHAW,  H.  B.,   1916.     Self-,  close-,  and  cross-fertilization  of  beets.     Mem. 

of  N.  Y.  Bot.  Gard.,  6:  149-152. 
SHAW,  J.  K.,  and  NORTON,  J.  B.,  1918.     The  inheritance  of  seed-coat  color 

in  garden  beans.     Massachusetts  Agr.  Exp.  Sta.,  Bull.  185:  59-104. 
SHULI-,  G.  H.,  1908.     The  composition  of  a  field  of  maize.     Am.  Breeders' 

Assoc.  Rept.,  4:  296-301. 

1909.     A  pure  line  method  of  corn  breeding.     Am.  Breeders'  Assoc. 

Rept.,  5:  51-59. 

1915.     Genetic  definitions  in  the  new  standard    dictionary.     Amer. 

Nat.,  59:  52-59. 
SMITH,  L.  H.,  1912.     Occurrence  of  natural  hybrids  in  wheat.     Proc.  Am. 

Breeders'  Assoc.,  Rept.  7  &  8:  412-414. 
So,   M.,  and  IMAL,  Y.,   1918.     On  the  xenia  of  the  barley.     Bot.  Mag., 

Tokyo,  32:205-214. 


LITERATURE  CITATIONS  315 

SPILLMAN,  W.  J.,  1901.  Quantitative  studies  in  the  transmission  of  parental 
characters  to  hybrid  offspring.  U.  S.  Dept.  Agr.,  Office  Exp.  Sta., 
Bull.  115,  88-97. 

1909.     The   hybrid   wheats.     Washington   Agr.    Exp.   Sta.,   Bull.  89, 
3-27. 
1911.     Inheritance  of  the  "eye"  in  Vigna.     Am.  Nat.,  45 :  513-523. 

1913.  Color  correlation  in  cowpeas.     Science  N.  S.,  38 :  302. 
SPRAGG,  F.  A.,  1919.     Robust  beans.     Michigan  Agr.  Exp.  Sta.,  Quart. 

Bull.  1:  173-174. 
SPRAGG,  F.  A.,  and  CLARK,  A.  J.,  1916.     Red  Rock  wheat.     Michigan  Agr. 

Exp.  Sta.    Cir.  31,  7  pp. 
STAKMAN,  E.  C.,  HAYES,  H.  K.,  AAMODT,  OLAF  S.,  and  LEACH,  J.  G.,  1919. 

Controlling   flax   wilt   by   seed   selection.     Jour.    Am.    Soc.   Agron., 

11:291-298. 
STAKMAN,   E.  C.,  LEVINE,  M.  N.,  and  LEACH,  J.  G.,  1919.     New  biologic 

forms  of  Puccinia  graminis.     Jour.  Agr.  Res.,  16:  103—105. 
STEWART,  F.  C.,  1916.     Observations  on  some  degenerate  strains  of  potatoes. 

New  York  (Geneva)  Agr.  Exp.  Sta.,  Bull.  422:  319-357. 
STEWART,  J.  B.,  1908.     The  production  of  cigar-wrapper  tobacco  under 

shade  in  the  Connecticut  Valley.     U.  S.  Dept.  Agr.,  Bur.  Plant  Indust., 

Bull.  138,  31  pp. 
STEWART,  J.  P.,  1912.     Factors  influencing  yield,  color,  size,  and  growth  in 

apples.     Ann.  Rept.  Pennsylvania  State  College  for  1910-11 : 401-492. 
STOUT,  A.  B.,  1915.     The  establishment  of  varieties  in  Coleus  by  the  selec- 
tion of  somatic  variations.      Carnegie  Inst.  of  Wash.,  Pub.  218:  1-80. 

1920.     Further  exper  mental  studies  on  self-incompatibility  in  herma- 
phrodite plants.     Jour.  Genetics,  9 :  104. 
STUART,  WM.,  1915.     Potato  breeding  and  selection.     U.  S.  Dept.  Agr., 

Bur.  Plant  Indust.,  Bull.  195,  35  pp. 
STURTEVANT  E.  L.,  1899.     Varieties  of  corn.     U.  S.  Dept.  Agr.,  Office  Exp. 

Sta.,  Bull.  57,  51-108. 
SURFACE,  F.  M.,  1916.     Studies  on  oat  breeding.     III.  On  the  inheritance 

of  certain  glume  characters  in  the  cross  A.  fatua  X  A.  sativa,  variety 

Kherson.     Genetics,  1 :  252-286. 
SURFACE,  F.  M.,  and  ZINN,  JACOB,   1916.     Studies  on  oat  breeding.     IV. 

Pure  line  varieties.     Main  Agr.  Exp.  Sta.,  Bull.  250:  97-148. 
SUTTON,  A.  W.,  1908.     Brassica  crosses.     Jour,  of  Linn.  Soc.,  38 :  337-349. 
SUTTON,  G.  L.,  1910.     Varieties  of  wheat  recommended  by  the  Department 

of  Agriculture.     Agr.  Gaz.  of  N.  S.  Wales,  21:  185-193. 
SUTTON,  IDA,  1918.     Report  on  tests  of  self-sterility  in  plums,  cherries,  and 

apples  at  the  John  Innes  Horticultural  Institution.     Jour.  Genetics, 

7:281-300. 
SWINGLE,  W.  T.,   1913.     Some  new  citrous  fruits.     Am.  Breeders'  Mag., 

4:83-95. 
TAMMES,  TINE,  1911.     Das  Verhalten  fluktuierend  variierender  Merkmale 

bei  der  Bastardierung.     Rec.  Trav.  Bot.  Neerl.,  8:  201-288. 

1914.  The  explanation  of  an  apparent  exception  to  Mendel's  law  of 
segregation.     Koninklyke  Akademe  van  Wetenschappen.     Te  Amster- 
dam., 16:  1021-1031. 


316  BREEDING  CROP  PLANTS 

1915.  Die   genotypische    Zusammensetzung   einiger   Varietaten   der- 
selben   Art  und    ihr  genetischer    Zusammenhang.     Rec.    Trav.    Bot. 
Neerl.,     12:217-277. 

1916.  Die  gegenseitige  Wirkung  genotypischer  Faktoren.     Rec.  Trav. 
Bot.  Neerl.    13:44-62. 

TERAO,  H.,  1918.     Maternal  inheritance  in  the  soybean.     Am.  Nat.,  62: 

51-56. 
THATCHER,  R.  W.,  1912.     Dominant  and  recessive  characters  in  barley  and 

oat  hybrids.     Proc.  Soc.  Prom.  Agr.  Sci.,  33:  37-50. 
THOMPSTONE,  E.,  1915.     Some  observations  on  upper  Burma  Paddy  (grown 

under  irrigation).     Agr.  Jour.,  India,  10:  26-53. 
TISDALE,  W.  H.,  1916.     Relation  of  soil  temperature  to  infection  of  flax  by 

Fusarium  lini.     Phytopath.,  6  :  412-413. 

1917.  Flax  wilt.     A  study  of  the  nature  and  inheritance  of  wilt 
resistance.     Jour.  Agr.  Res.,  11:  573-605. 

TOWNSEND,  C.  O.,  1909.     Breeding  sorghum.     Am.  Breeders'  Assoc.  Rept., 

5 :  269-274. 
TRABUT,    L.,    1914.     The   origin   of  cultivated   oats.     Jour.  Heredity,  5: 

56-85. 
TSCHERMAK,  E.  VON,  1901.     Ueber  Ziichtung  neuer  Getreiderassen  mittelst 

ktinstlicher    Kreuzung.     Zeitschr.   f.   d.  Land.   Versuch.  in  Oest.,  4: 

1029-1060. 

1906.     tlber     Ziichtung    neuer     Getreiderassen     mittels    kiinstlicher 

Kreuzung.  II.    Mitteilung.     Kreuzungstudien  am  Roggen.     Zeitschr. 

f .  d.  Landw.  Versuch.  in  Oest.,  9 :  699-743. 

1913.     Uber  seltene  Getreidebastarde.     Beitrage  zur '  Pflanzenziicht., 

3:49-61. 

191 4a.     Die  Verwertung  der  Bastardierungfiirphlogenetische  Fragenin 

der  Getreidegruppe.     Zeitschr.  fur  Pflanzenzucht.,  2 :  303-304. 

19146.     tTber  die  Vererbungsweise  von  Art-  und  Gattungs-bastarden 

innerhalb    der    Getreidegruppe.     Mitt.    Landw.    Lehrkanzeln    K.    K. 

Hochschule  fur  Bodenkultur  in  Wien,  2 :  763-772. 

1916.     tlber  den  gegenwartigen  Stand  der  Gemusezlichtung.     Zeitschr. 

fur  Pflanzenzucht.,  4:  65-104. 
ULRICH,  K.,  1902.     Die  Bestaubung  und  Befruchtung  des  Roggens.     Inaug. 

Diss.  Halle. 
VALLEAU,  W.  D.,  1918.     Sterility  in  the  strawberry.     Jour.  Agr.  Res.,  12: 

613-669. 
VILMORIN,  Louis  DE,  1852.     Note  sur  la  creation  d'une  nouvelle  race  de 

betterave  a  sucre.,  pp.  25-29.     Notices  sur  L' Amelioration  des  Plantes 

by  Louis  Leveque  de  Vilmorin  and  Andre  Leveque  de  Vilmorin.     Vilmo- 

rin-Andrieux,  Paris. 
VILMORIN,   M.   H.,  1880.     Essais  de    croisement    entres    bles    differents. 

Bull.  Soc.  Bot.  de  France,  27:  356-361. 

1883.     Exper  ences  de   croisement    entres    des  bles  differents.     Bull. 

Soc.  Bot.  de  France,  30 :  58-63. 
VILMORIN,    P.     DE,    1910.     Recherches    sur    1'heredite  mendelienne.     In 

Compt.  Rend.  Acad.  Sci.,  151:548-551. 
WAITE,  M.  B.,  1905.     Sterility  in  Japanese  plums.     Am.  Agric.,  75:  112. 


LITERATURE  CITATIONS  317 

WALDRON,  L.  R.,  1912.     Hardiness  in  successive  alfalfa  generations.     Am. 

Nat.,  46 : 463-469. 

1919.     Cross-fertilization    in    alfalfa.     Jour.    Am.    Soc.    Agron.,  11: 

259-266. 

WALLER,  A.  E.,  1917.     A  method  for  determining  the  percentage  of  self- 
pollination  in  maize.     Jour.  Am.  Soc.  Agron.,  9:  35-37. 
WALTER,    H.    E.,    1916.     Genetics.     The    Macmillan    Co.,    New   York, 

272  pp. 
WARBURTON,  C.  W.    1914.     Notes  on  oats  for  the  Southern  United  States. 

Jour.  Am.  Soc.  Agron.,  6:  118-121. 
WAUGH,  F.  A.,  1896.     The  pollination  of  plums.     Vermont  Agr.  Exp.  Sta., 

Bull.  63 :  47-65. 

1897.  Problems    in    plum-pollination.     Vermont     Agr.     Exp.     Sta. 
Ann.  Kept.,  10:  87-98. 

1898.  Problems  in  plum-pollination.     Vermont  Agr.  Exp.  Sta.    Ann. 
Kept.,  11 :  238-262. 

1899.  The  pollination  of  plums.     Vermont    Agr.     Exp.     Sta.     Ann. 
Kept.,  12:189-209. 

1900.  Further    work     in     plum-pollination.     Vermont     Agr.     Exp. 
Sta.  Ann.  Kept.,  13:  355-362. 

1901.  Plums    and    plum    culture.     Orange     Judd    Co.,    New    York, 
371  pp. 

WEBBER,  H.  J.,  1900.     Xenia,  or  the  immediate  effect  of  pollen,  in  maize. 

U.  S.  Dept.  Agr.,  Div.  Veg.  Phys.  &  Path.,  Bull.  22,  38  pp. 

1905.     Cotton  breeding.     In  Am.  Breeders'  Assoc.  Kept.,  1:  37-44. 

1911.     Preliminary  notes  on  pepper  hybrids.     Amer.  Breeders'  Assoc. 

Kept.,  8:  188-199. 
WEBBER,  H.  J.,  HUNT,  T.  F.,  GILMORE,  J.  W.,  CLARK,  C.  F.,  and  FRASER, 

S.,  1912.     The  production  of  new  and  improved  varieties  of  timothy. 

New  York  Cornell  Agr.  Exp.  Sta.,  Bull.  313:  339-392. 
WEBBER,  H.  J.  and  ORTON,  W.  A.,  1902.     A  cowpea  resistant  to  root-knot. 

Part  II.     U.  S.  Dept.  Agr.,  Bur.  Plant  Indust.,  Bull.  17,  23-36. 
WELLINGTON,  R.,  1912.     Influence  of  crossing  in  increasing  the  yield  of  the 

tomato.     N.  Y.  Agr.  Exp.  Sta.,  Bull.  346:  57-76. 

1913a.     Studies  of  natural  and  artificial  parthenogenesis  in  the  genus 

Nicotiana.     In  Am.  Nat.,  47,  279-306. 

19136.    Raspberry  breeding.     Soc.  Hort.  Sci.  Rept.  for  1913:  155-159. 
WESTGATE,  J.  M.,  1910.     Variegated  alfalfa.     U.S.  Dept.  Agr.,  Bur.  Plant 

Indust.,  Bull.  169:  1-63. 

1915.  Red-clover  seed  production:  pollination  studies.     U.  S.  Dept. 
of  Agr.,  Bull.  289,  31  pp. 

WHITE,   O.   E.,    1910.     Inheritance  studies  in   Pisum.     I.  Inheritance  of 
cotyledon  color.     Am.  Nat.,  60 :  530-547. 

1916.  The  origin  and  history  of  our  more  common  cultivated  fruits. 
Brooklyn  Bot.  Gard.  Leaflets,  Series  4:  pp.  1-12. 

1917.  Inheritance  of  endosperm  color  in  maize.     Am.  Jour.  Botany, 
4:396-406. 

1917.     Inheritance  studies  in  Pisum.     IV.  Interrelation  of  the  genetic 
factors  of  Pisum.     Jour.  Agr.  Res.,  11:  167-190. 


318  BREEDING  CROP  PLANTS 

1918.     Inheritance  studies  in  Pisum.     III.  The  inheritance  of  height 
in  peas.     Memoir  of  the  Torrey  Bot.  Club,  17 :  316-322. 
WIGHT,  W.  F.,  1915.     Native  American  species  of  prunes.     U.  S.  Dept.  of 
Agr.,  Bull.  179:  1-75. 

1915.  The  varieties  of  plums  derived  from  native  American  species. 
U.  S.  Dept.  of  Agri.,  Bull.  172:  1-44. 

1916.  Origin,  introduction  and  primitive  culture  of  the  potato.     Proc. 
Nat.  Potato  Assoc.  of  America,  3 :  35-52. 

WILLIAMS,  C.  G.,  1905.     Pedigreed  seed  corn.     Ohio  Agr.  Exp.  Sta.,  Cir. 

42:  1-11. 

1907.     Corn  breeding  and  registration.     Ohio  Agr.  Exp.  Sta.,  Cir.  66: 

1-14. 

1916.     Wheat  experiments.     Ohio  Agr.  Exp.  Sta.,  Bull.  298:  449-484. 
WILLIAMS,C.  G.  and    WELTON,   F.  A.,  1911.     Wheat  experiments.     Ohio 

Agr.  Exp.  Sta.,  Bull.  231,  22  pp. 

1913.     Oats.     Ohio  Agr.  Exp.  Sta.,  Bull.  257 :  255-283. 

1915.     Corn  experiments.     Ohio  Agr.  Exp.  Sta.,  Bull.  282:  84-91. 
WILSON,   A.    D.,   and   WARBURTON,    C.   W.,  1919.     Field   Crops.     Webb 

Pub.  Company,  St.  Paul,  Minn.,  509  pp. 
WILSON,  J.  H.,  1916.     Further  experiments  in  crossing  potatoes.     Trans. 

High.  Agr.  Soc.  Scotland.     Fifth  series,  28 : 33-55. 
WITTE,   H.,  1919.     Breeding  timothy  at  Svalof.     Jour.  Heredity,  10:   291- 

299. 
WITTMACK,  L.,  1909.     Studien  iiber  die  Stammpflanzen  der  Kartoffel. 

Ber.  Deutschen  Bot.  Gesell.,  27 : 28-42. 

WOOD,  T.  B.,  and  STRATTON,  F.  J.  M.,  1910.     The  interpretation  of  experi- 
mental results.     Jour.  Agr.  Sci.,  3  :  417-440. 
WOLFE,  T.  K.,  1915.     Further  evidence  of  the  immediate  effect  of  crossing 

varieties    of    corn    on    the    size    of  seed  produced.     Jour.  Am.    Soc. 

Agron.,  7:265-272. 
ZINN,  JACOB,  and  SURFACE,  F.  M.,  1917.     Studies  on  oat  breeding.     V.  The 

FI  and  F2  generations  of  cross  between  a  naked  and  a  hulled  oat. 

Jour.  Agr.  Res.,  10 :  293-312. 


INDEX 


Aaronsohn,  wild  wheat,  78 
Alfalfa,  flower  structure,  217 

Grimm  variety,  216 

origin  of,  215 

pollination  in,  40,  207 

selection  in,  293 

winter  resistance,  218 
Alkemine,  rice  pollination,  37 
Allard,  tobacco,  inheritance  in,  161 

tobacco  mutations,  167 
Apples,  self -sterility  in,  269,  271 

inheritance  in,  271 
Arny  and  Garber,  weight  of  seed  and 

plant  vigor,  123 
Arny  and  Hayes,  border  effect  in 

plot  tests,  61 
Asparagus,  pollination  in,  253 

rust  resistance,  253 


B 

Babcock  and  Clausen,  crossing  over, 
27 

homozygosis  following  a  cross, 
117 

selection  in  sugar  beets,  119 
Backhouse,  wheat  species  crosses,  79 

wheat-rye  crosses,  106 
Bailey,  crosses  in  cucurbits,  256 

evolution  in  fruits,  264 

inheritance  in  raspberries,  272 

sweet  corn,  origin,  235 
Bain   and   Essary,   clover   anthrac- 

nose,  215 

Ball,  pollination,  grain  sorghums,  38 
Balls,  cotton,  natural  crosses,  38,  173 

cotton,  inheritance,  175 
Barley,  awn  in  relation  to  yield,  104 

classification,  98 


Barley,  independent  Mendelian  in- 
heritance, 102 

inheritance  of  spike  density,  28, 
103 

pollination  in,  36 

species  crosses,  99 

winter  vs.  spring  habit,  103 

xenia  in,  103 
Barrus,   disease  resistance  in  bean, 

244 

Beach,  self-sterility  in  grape,  269 
Beal,  varietal  crosses  in  maize,  202 
Bean,  classification,  241 

disease  resistance  in,  139,  244, 
245 

flower  structure,  243 

inheritance  of  characters,  242, 
244 

M.  A.  C.  Robust,  131 
Beet,  inheritance  and  breeding,  250 
Belling,  inheritance  in  velvet  bean, 
149 

varietal  crosses,  maize,  203 
Biffin,  barley  inheritance,  102,   103 

disease  resistance  in  wheat,  85, 
134 

immediate  effect  of  pollination 
in  wheat,  82 

linkage  in  wheat  crosses,  79 

polonicum-wheat  crosses,  79 

species  crosses,  barley,  101 

wheat  pollination,  35 
Bolley,  flax,  wilt  resistance,  157,  286 
Brand,  alfalfa,  Grimm,  216 
Brassica,  inheritance  studies  in,  251 
Breeding,  asparagus,  254 

beets,  250 

by  crossing,  116 

by  selection,  115 

cabbage,  252 

clover,  215 


319 


320 


INDEX 


Breeding,  cotton,  177 

cucurbits,  259 

flax,  158 

fruits,  266,  273,  278,  279 

Hays,  progeny  test,  111 

Hopkins,  ear-to-row  test,    111, 
198 

keeping  records  in,  111 

maize,  196,  290 

radish,  250 

self-fertilized  vegetables,  247 

small  grains,  111 

soybeans,  148 

timothy,   Cornell  station,   210, 

211 
at  Svalof,  212 

wheat,  135,  137 
Buckwheat,  breeding,  108 

characters  of,  107 

original  home  of,  107 

species,  107 

Bull,  the  Concord  grape,  265 
Burnett,  oat  selections,  131 
Bushnell,  squash,  selfing  in,  68 
breeding,  260 

C 

Camerarius,  on  sexuality,  3 
Caporn,  inheritance,  size  characters 

oats,  96 

Carleton,  origin  of  rice,  108 
Carrier,  increase  of  seed  size  due  to 

crossing,  201 
Check  plots,  correcting  yields  by,  53, 

55 

estimating  heterogeneity  by,  53 
in  obtaining  probable  errors,  55 
Cherries,  self-sterility  in,  269 
Citranges,  277 

Clover,  pollination  in,  207,  214 
selection  in,  292 
species,  214 

Coe,  mutations  in  velvet  beans,  150 
Collins,  origin  of  maize,  182 

podded  characters  in  maize,  189 
varietal  crosses  in  maize,  202, 

203 

Collins    and    Kempton,    endosperm 
characters  in  maize,  185 


Collins   and   Kempton,  increase  in 
seed  size  due  to  a  cross,  201 
Competition  in  variety  test,  63 

in  rod  row  tests,  64 
Correlations,  morphological  charac- 
ters    and     lodging,     small 
grains,  127 

value  of,  125 

Correns,    endosperm    characters    of 
maize,  185 

xenia  in  maize,  183 
Cotton,  chromosome  number,   177 

cultivated  species,  173 

disease  resistance,  177,  178 

flower  structure,  174,  175 

inheritance  in,  176 

mutations  in,  177 

natural  crosses  in,  38,  173 

origin  of,  173 
Cowpea,  inheritance  in,  143,  144 

Iron,  a  variety  of,  131 

origin,  143 

resistance  to  disease,  144,  145 

selection  and  crossing   results, 

145 

Crane,  inheritance  in  tomato,  245 
Crop  improvement,  value  of,  14,  15 
Crops,  mode  of  reproduction  of,  33, 

34 

Crosses,  handling  of,  116 
Crossing,  artificial,  68 

small  grains,  69 

technic  of,  74 
Crossing-over,  12,  27 
Cucumber,  heterosis  in,  257 

inheritance,  257 
Cucurbit  crosses,  7 

classification,  255 

Cytology,     reduction     division     in 
plants,  17,  18 


1) 


Darwin,  heredity,  10 

natural  selection,  9 

variability,  10 
Date  palm,  sexuality,  3 
De  Candolle,  origin  and  antiquity  of 
vegetables,  235 


INDEX 


321 


Dillman,  selection  in  sorghum,  179 
Disease  resistance,  in  asparagus,  253 
in  beans,  139,  244 
in  cabbage,  252 
in  clover,  215 
in  cotton,  177 
in  cowpeas,  144,  145 
in  flax,  151 
in  oats,  95 
in  rice,  109 
in  timothy,  213 
in  tomatoes,  248 
in  watermelons,  258 
in  wheat,  76,  85,  134 
Dorsey,  breeding  fruits,  279 
mutations  in  fruits,  264 
origin  of  fruits,  263 
self-fertile  and  self-sterile  fruits, 

269 

self-sterility  in  grape,  plum,  270 
Durst,    disease   resistant   tomatoes, 
248 

E 

East,  artificial  crossing,  68 

maize,  ear-to-row  tests,  198 

self-fertilization  in,  202 
mathematical    requirements    of 

size  inheritance,   163,  164 
potato,  clonal  selection,  228 
introduction  to  Europe,  220 
pollination  in,  224,  225 
species  in,  219 

tobacco,  color  of  flowers,  161 
size  of  flower,  165 
species  crosses,  159 
sterility  in,  160 

East  and  Hayes,  hybrid  vigor,  45,  47 
maize,  inheritance,  185,  187 
pericarp  color,  188 
podded  character,  189 
seed    and    ear    characters, 

192 

subspecies  of,  182 

East  and  Jones,  factor  stability,  32 
loss  mutations,  276 
size  inheritance,  30 
East  and  Park,  sterility  in  tobacco, 
160 

21 


Edgerton,  disease  resistance  in  toma- 
toes, 248 

Emerson,  bean,  inheritance  in,  242 
maize,  auricle  and  ligule,  189 
cob  color,  188 
endosperm  characters,  187 
squash,  size  inheritance  in,  258 
tomato,  inheritance,  246 
Emerson  and  East,  size  inheritance, 

192 
Engledow,  linkage  in  wheat  crosses, 

79 
Etheridge,  oat  classification,  89 


Failyer    and    Willard,    selection    in 
sorghum,  179 

Farrar,  wheat  breeding,  135 

Fertilization,  maize,  19 

Field  plot  technic,  65,  266 

Flax,  breeding,  158 

flower  structure,  154 
inheritance,  155,  156 
natural  crosses,  37 
origin  of,  153 
species  crosses,  153 
wilt  resistance,  158,  287 

Fletcher,  self-sterility  in  pears,  269 
strawberry,  varieties,  262 

Focke,  xenia,  183 

Frear,  crossing  in  uncovered  wheat 
spikes,  69 

Freeman,  linkage  in  wheat,  80 
seed  characters  of  wheat,  82 
sterility  in  wheat  crosses,  79 

Fruits,  crosses  in,  277 

early  breeders  of,  264 
origin,  antiquity  of,  261,  264 
self-sterility  and  heterozygosity 
in,  267 

Fruwirth,  barley  pollination,  36 
flax,  natural  crosses,  37 
maize,  self-fertilization,  39 
oats,  natural  crosses,  36 
potatoes,  inheritance  in,  221 
pure-line  selection,  12  • 
rye,  pollination  in,  40 
wheat,  pollination,  35 
winter-spring  habit,  barley,  103 


322 


INDEX 


G 

Gaines,  disease  resistance  in  wheat, 

86 

oat  crosses,  93 
seed  characters  in  wheat,  82 
spike  density  in  wheat,  81 
Gametes,    production    in    flowering 

plants,  17 

Garber,  stem  rust  in  oats,  95 
Garber  and  Olson,  stem  characters 

of  small  grains,  127 
Garner,  tobacco  mutations,  167 
Garner  and  Allard,  effect  of  length 
of   day   on   flowering,    169 
Genetics,  factor  stability,  32 
inheritance  factors,  20 
inheritance,  two  independently 

inherited  factors,  23 
linkage,  26 

method  of  studying,  16 
size  characters,  27 
Germ  plasm,  constancy  of,  10 
Gilbert,  potato  characters,  223 
Giltay,  xenia  in  rye,  106 
Goff ,  self-sterility  in  plum,  269 
Goodspeed,         parthenogenesis     in 

tobacco,  160 

size  inheritance  in  tobacco,  165 
Goss,  early  pea  crosses,  7 
Graham,    inheritance    in    sorghum, 

179 

pollination  of  sorghum,  38 
Grape,  early  breeding,  265 
inheritance  in,  272 
self-sterility  in,  269 
Grasses,  pollination  in,  41,  207 
economic  species,  207 
selection  in,  293 
Griff ee,  FI  wheat  crosses,  43 
Groth,  inheritance  in  tomato,  245 
Guignard,  double  fertilization,  183 

H 

Hagedoorn,  early  wheat  selections, 

119 
Hallet,     early    selections    in    small 

grains,  118 
Hansen,  fruit  breeding,  278 


Harlan,  barley,  natural  crosses,  36 

classification  of  barley,  98 
Harlan    and    Anthony,    the    barley 

awn,  104 
Harlan  and   Hayes,   barley  species 

crosses,  99 
Harland,  cowpea,  inheritance,   143, 

144 

natural  crosses,  39 
Harris,  soil  heterogeneity,  51 
Harshberger,  home  of  maize,  181 
Hartley  and  others,  varietal  crosses 

of  maize,  203 
Hasselbring,  pure  lines  in  tobacco, 

166 

Hayes,  maize,  self-pollination,  39 
pericarp  color,  maize,  188 
pure-line  selection,  tobacco,  122, 

166 

size  characters,  165 
tobacco  mutations,  167 
Hayes  and  Arny,  replication,  59 
Hayes  and  Beinhart,  tobacco  muta- 
tions, 167 
Hayes  and  East,  maize  inheritance, 

185,  192 

maize,  varietal  crosses,  203 
Hayes,    East    and    Beinhart,    size 
characters,    tobacco,     162, 
165 
Hayes  and  Garber,  winter  hardiness 

in  wheat,  87,  138 
high  protein  maize,  195 
Hayes    and    Harlan,    barley,    spike 

density,  104 
Hayes  and  Jones,  FI  tomato  crosses, 

43 
Hayes  and  Olson,  FI  varietal  maize 

crosses,  203 
Hayes  and  others,  disease  resistance 

in  wheat,  85 
sterility  in  wheat,  79 
Hayes  and   Stakman,  rust  in  timo- 
thy, 213 

Hays,  small  grain  breeding,  112 
Head  thrasher,  small  grains,  117 
Heckel,  mutations  in  potatoes,  220 
Hector,  inheritance  in  rice,  109 
natural  crosses  in  rice,  37 


INDEX 


323 


Hedrick  and  Anthony,   inheritance 

in  grape  and  raspberry,  272 

Hedrick  and  Wellington,  inheritance 

in  apples,  268,  271 
Henkemeyer,    chaff    characters    in 

wheat,  84 

Heterogeneity,  in  field  plots,  51 
estimating,  53 

inheritance  in  raspberry,  272 
Hildebrand,  rye  pollination,  39 
Hilson,  inheritance  in  sorghum,  178 
Homozygosis,  from  self-fertilization, 

49 

Hopkins,  ear-to-row  maize,  198 
Hoshino,   inheritance   in   rice,    109, 

110 

Houser,  Fi  tobacco  crosses,  42 
Howard    and    others,     inheritance, 

beards  in  wheat,  85 
chaff  characters  in  wheat,  84 
seed  characters  in  wheat,  82 
standing  power  in  wheat,  88 
natural  crosses,   in  beans   and 

peas,  39 
flax,  37 
tobacco,  36 
wheat,  35 

parthenogenesis  in  tobacco,  160 
tobacco  groups,  159 
Hutcheson,  pure-line  selection,  122 
Hutcheson  and  Wolf,  maize  varietal 

crosses,  203 
Hybrid  vigor,  47 


Ikeno,  inheritance,  in  rice,  108,  109 
inheritance,   in  peppers,    247 
natural  crosses  in  rice,  37 


Jardine,  Kanred  wheat,  76,  128 
Jellneck,  artificial  wheat  crosses,  71 
Jenkins,  number  of  seeds  per  tobacco 

plant,  36 

Jensen,  plant  height,  tobacco,  165 
Jesenko,  wheat-rye  crosses,  106 
Johannsen,  the  pure  line,  11,  120 


Johnson,  cytology  of  fertilization,  5 
sex  in  date  palm,  3 
tobacco  breeding,  140 
tobacco  inheritance,  165 . 
Jones,  crossing  selfed  strains,  maize, 

205 

heterosis,    49 
homozygosis  on  selfing,  49 
increased  seed  size  in  a  cross, 

201 
inheritance,         pseudo-starchy, 

maize,  184 

lethal  endosperm,  maize,  185 
tomato,  246 

self-fertilization,  maize,  47 
Jones  and  Gallastegui,  podded  char- 
acter, maize,  189 

Jones    and    Oilman,    disease   resis- 
tance, cabbage,  252 
self -sterility,  cabbage,  251 
Jones  and  others,   varietal  crosses, 
maize,  203 

K 

Kajanus,  inheritance  in  beet,  250 
Karper    and    Conner,     pollination, 

grain  sorghums,  38 
Kearney,  cotton  mutations,  177 
Keeble  and   Pellew,   inheritance   in 

pea,  236 
Kezer  and  Boyack,   wheat  crosses, 

79,  80 
Kiesselbach,  competition  in  variety 

tests,  63  • 

ear-to-row  breeding,  maize,  199 
varietal  crosses,  maize,  203 
Kiesselbach  and  Ratcliff,  selection  in 

oats,  130 

Kirchner,  wheat  pollination,  35 
Knight,  early  crosses,  6 
fruit  breeding,  264 
self -sterility  in  apple,  271 
Knuth,  maize  pollination,  39 
Koch,  inheritance  in  rice,  109,  110 
Koelreuter,  early  crosses,  5 


Leake,  cotton  inheritance,  175 
cotton,  natural  crosses,  38 


324 


INDEX 


Le  Couteur,  selection,  small  grains, 

119 

Legumes,  crossing  artificially,  72 
Leidigh,     self-fertilizing     sorghums, 

180 

Leighty,  wheat-rye  crosses,  35,  106 
Leighty  and   Hutcheson,   blooming 

in  wheat,  35,  69 
Lewis    and    Vincent,    heterosis    in 

apples,  269 

self-sterility  in  apples,  269 
Lily,  anther  and  pollen  of,  18 
Lindstrom,  chlorophyll  inheritance, 

maize,  190 

Linkage,  in  cotton,  175 
in  oats,  97 
in  pea,  241 
in  tomato,  246 
in  wheat,  79,  80,  84 
of  characters,  26 
Love  and  Craig,  pure-line  selections, 

oats,  123 

chaff  characters,  wheat,  84 
oat  crosses,  92,  93,  94 
rod-row  method,  114 
thrashing  machine,  117 
wheat,  species  crosses,  78 
wheat-rye  crosses,  106 
Love  and  McRostie,  hulled  vs.  hull- 
less,  oats,  93 

Lumsden,  inheritance  in  muskmelon, 
257 


M 


MacDougal,  inheritance  in  potatoes, 
222 

Macoun,  degeneracy  in  potato,  230 
hardy  fruits,  277 

Maize,  chemical  composition,  193 
chlorophyll  inheritance,  190 
ear  characters  and  yield,  197 
ear-to-row  breeding,  198 
effects  of  self-fertilization,  46,  47 
endosperm     character     inheri- 
tance, 185 
fertilization  in,  19 
heterosis  in,  200 
home  grown  seed,  199 


Maize,    increase   in   seed   size   from 

crossing,  201 
near  relatives  of,  181 
origin  of,  182 
plant     character     inheritance, 

187-196 

podded  condition  of,  189 
Malte    and    Macoun,    crossing    in 

vegetables,  251 
Martin,  clover,  pollen  germination, 

215 
Mayer-Gmelin,     spike     density     in 

wheat,  81 

McFadden,  wheat-rye  crosses,    106 
McRostie,  beans,  disease  resistance, 

139,  245 

Mendel's  law,  12 
Mercer  and  Hall,  replication  in  plot 

test,  59 

size  and  shape  of  plot,  60,  61 
Montgomery,  ear-to-row  maize,  198 
origin  of  maize,  181 
rod-row  method,  115 
size  of  plot,  60 
weight  of  seed  planted,  126 
Morrow    and    Gardner,    varietal 

crosses,  maize,  202,  203 
Munson,  evolution  of  fruits,  264 
Muskmelon,  inheritance  in,  257 
Mutations,  cotton,  177 

due  to  loss  of  factors,  276 
fruits,  273 
maize,  189 
potatoes,  220,  229 
tobacco,  167 
velvet  beans,  150 


X 


Nabours,    cross-overs   in   partheno- 
genesis, 276 
Natural  selection,  9 
Naudin,  on  segregation,  7 

on  cucurbit  classification,  254 
Nawaschin,  double  fertilization,  183 
New  introductions,  113 
Newman,  correlations,  125 

improvement  by  crossing,  134 

selection  methods,  127 


INDEX 


325 


Nilsson,  inheritance  in  potatoes,  221 
oat,  classification,  95 
potato,  degeneracy  in,  232 
rye,  self-pollination,  40 
Nilsson-Ehle,    oat    inheritance,    92, 

93,  95,  96,  97,  133 
oats,  false  wild,  97 
wheat,  disease  resistance,  85 
seed  characters  in,  82 
spike  density  in,  81 
winter-hardiness,  87 
Norton,  A.  sterilis,  oats,  90 

rust  resistance,  asparagus,  254 
time  of  blooming,  oats,  71 


0 


Oats,  awn  development,  90 

color  of  lemma,  93 

color  of  stem,  92 

disease  resistance,  95 

false  wild,  97 

improvement  by  crossing,  133 

linkage  in,  97 

natural  crosses,  36 

open  vs.  side  panicle,  95 

origin  of,  90 

pubescence  on  grain,  94 

selection  in,  129,  130,  131 

size  characters,  96 

species  groups,  89 

yellow  factor,  92 
Oliver,  crossing  methods,  71 
Olson,  Bull  and  Hayes,  ear  charac- 
ters and  yield,  maize,  198 
Origin  and  development  of  crops,  1 
Orton,  cotton,  wilt  resistance,  177 

cowpea,  resistance  and  wilt  and 
root-knot,  144 

watermelon,  wilt  resistance,  258 


Pea,  classification  of,  236 

genetic  factors  in,  240 

inheritance  in,  236 

linkage  in,  241 

natural  crosses,  38 

structure  of  flower,  237 
Pear,  self -sterility  in,  269 
Pearl  and  Miner,  probable  errors,  56 
Pepper,  characters,  246 

inheritance  in,  247 
Piper,  alfalfa,  origin  and  species,  215 

cowpea,  breeding,  146 

cowpea,  origin  of,  143 

pea,  natural  crosses,  38 

sorghum  species,  178 

soybeans,  natural  crosses,  39 
Piper  and  Morse,  origin  of  soybean, 

146 
Piper  and  others,  alfalfa  pollination, 

40,  216 

Plant  breeding,  development  of  art, 
2 

relation  of  biological  principles 

to,  8 

Plum,  self-sterility  in,  269 
Pollination,  artificial  crosses,  oats,  71 

artificial  self-pollination,  67 

depollination  with  water,  73 

tools  for,  73 

Pope,  pollination  of  crop  plants,  34 
Price,  cabbage,  self -sterility,  251 

inheritance  in  cabbage,  251 
Price  and  Drinkard,  tomato  inheri- 
tance, 245 

Pridham,  oat  crosses,  36,  93 
Probable  error,  use  of,  56 

in  eliminating  strains,  57 

pairing  method  of  obtaining,  57 
Pure  line  theory,  11,  120,  121 


Pammel,  crosses  in  cucurbits,  256 
Parker,  disease  resistance  in  oats,  95 

spike  density  in  wheat,  81 
Parnell  and  others,   inheritance   in 
rice,  109 

natural  crosses  in  rice,  37 


Quanjer,  diseases  in  potatoes,  231 


R 


Radish,    origin,    inheritance,    breed- 
ing, 249 
Raspberry,  inheritance,  272 


326 


INDEX 


Ray,  species,  first  use  of  term,  9 
Replication,  value  of,  58 

correct  method  of,  60 

reducing  probable  error  by,  59 
Rice,  important  characters  of,  108 

inheritance  summary,  109 

natural  crosses,  37 

origin,  108 
Rimpau,  barley  pollination,  36 

oat  crosses,  36 

Riolle,  radish  inheritance,  249 
Roberts,    on   early   plant   breeders, 

5-8 

Roguing,  seed  plots,  180     . 
Riimker,  von,  xenia  in  rye,  106 

inheritance  in  rye,  106 
Russell      and      Morrison,      natural 

crosses,  soybean,  39 
Rye,  pollination,  39,  106 

winter  vs.  spring,  106 

xenia  in,  106 


S 


Salaman,  potato  inheritance,  221 
Salmon,  blooming  in  wheat,  69 
Sargaret,     cucurbits,    classification, 
254 

cucurbits,  crosses  in,  7 
Saunders,  Marquis  wheat,  136 

wheat  crosses,  36 
Scott,  evolution,  8 
Seed,  improved  corn,  287 

pure  bred,  281 

registered,  286,  293 

what  is  good,  282 

wilt  resistant  flax,  287 
Selection,  individual  plant,  119,  128 

in   small   grains,  early  workers, 
118,  119 

isolation  of  pure  lines,  125 

methods  of,  114,  115 

sorghum,  sugar  content,  179 
Self-fertilization,  in  normally  cross- 
fertilized  species,  45,  205 
Setchell,  tobacco  sections,  159 
Sexuality,  Camerarius  proves  fact,  3 

further  proof  of,  4 
Shamel,  tobacco  breeding,  165 


Shamel  and  Cobey,  tobacco  breed- 
ing, 166 
Shamel  and  others,  bud  selection  in 

fruits,  274 

Shaw,  self-fertilization  in  beet,  250 
Shaw    and    Norton,    inheritance    in 

beans,  242 

Shirreff,  early  selections,  118 
Shull,  self-fertilization  in  maize,  202, 

205 

Smith,    selection    for  chemical  con- 
tent, maize,  193 
Sorghum,  breeding,  179 
classification,  178 
inheritance  in,  178 
origin  of,  178 

self-fertilization  in,  38,  180 
Soybeans,  breeding,  148 
characters  of,  146 
inheritance  in,  147 
natural  crosses,  39 
origin  of,  146 

Species,  crosses,  Fi,  tobacco,  45 
crosses,  willow,  8 
first  use  of  term,  9 
Spillman,   cowpea  inheritance,    143, 

144 

wheat,  density  of  spike,  81 
Spragg,    M.    A.    C.    Robust    bean, 

131 
Spragg  and  Clark,  Red  Rock  wheat, 

128 

Squash,  breeding,  259 
flower  structure,  255 
size  inheritance,  258 
Stakman  and  others,  biologic  forms 

of  rust,  85 

flax  wilt  resistance,  157 
Stewart,  bud  selection  in  fruits,  275 

tobacco  breeding,  166 
Stout,  bud  variations,  274,  276 
Stuart,   potato  breeding,    223,    224, 

226 

Surface,  oat  inheritance,  92,  93 
oat  selections,  129 
oat  species  crosses,  90,  94 
Sutton,  Brassica  crosses,  251 
self-sterility,  fruits,  269 
Swingle,  citrus  fruits,  277 


INDEX 


327 


Tammes,  flax  inheritance,  155,  156 
flax  species  crosses,  153 

Thatcher,  barley  inheritance,  102 

Thompstone,  inheritance  in  rice,  109 
natural  crosses  in  rice,  37 

Timothy,  breeding,  210 
flower  structure,  209 
variability  in,  208 

Tisdale,  flax  wilt,  inheritance,  158 

Tobacco,  breeding,  140 

classification  in  groups,  159 
color  of  flowers,  inheritance,  161 
Fi  crosses,  vigor  of,  42 
mutations  in,  167 
natural  crosses  in,  36 
number  of  seeds  per  plant,  36 
parthenogenesis  in,  160 
quantitative  characters,  inheri- 
tance, 162 

Tolaas  and  Bisby,  disease-free  pota- 
toes, 291 

Tomato,  characters  of,  245 

Fi  crosses,  vigor  of,  42,  43 
inheritance,  245,  246 
natural  crosses,  39 

Townsend,  self-fertilization  in  sorg- 
hum, 180 

Trabut,  oats,  A.  sterilis  origin,  90 

Tschermak,  barley  inheritance,  102 
bean  inheritance,  242 
oats,  natural  crosses,  36 
oats,  species  crosses,  90 
pea,  inheritance,  238,  239 
rye,  wild  rye,  106 
rye,  winter  vs.  spring  habit,  106 
tomato,  inheritance,  245 
vegetable,  mass  selection,  250 
wheat,     species      groups     and 
crosses,  77,  78 

U 
Ulrich,  rye-pollination,  40 

V 

Valleau,  sterility  in  strawberry,  270 
Variations,  hybridization,  13 


Variations,  mutations,  21 

new  combination,  21 

non-heritable,  20 

Vegetables,     cross-fertilized    group, 
248 

origin  and  antiquity  of,  235 

self-fertilized  group,  234 
Velvet  bean,  breeding,  151 

characters  and  inheritance,  149 

mutations  in,  150 

origin,  149 
Vilmorin,  early  selections,  119 

selection  in  beets,  250 

wheat  species  crosses,  77 
Von  Mons,  selection  in  fruits,  264 
DeVries,  correlations,  value  of,  125 

individual     plant     method     of 
selection,  120 

wheat,  barley  and  oats  pollina- 
tion, 35 

xenia  in  maize,  183 


W 


Waite,  self-sterility  in  plum,  269 
Waldron,  alfalfa,  natural  crosses,  40 
pollination  of,  216 
winter  hardiness,  216 
Waller,    normal    self-pollination    in 

maize,  39 

Watermelon,  wilt  resistance  in,  258 
Waugh,  self-sterility  in  plum,  269 
Webber,  cotton  inheritance,  177 
cotton,  natural  crosses,  38,  173 
pepper  inheritance,  247 
timothy  breeding,  210 
xenia  in  maize,  183 
Webber  and  Orton,  cowpea,  disease 

resistance,  145 
Weigmann,  early  crosses,  5 
Weissmann,     constancy     of     germ 

plasm,  10 

Wellington,  Fi  tomato  crosses,  42 
heterosis   and   inheritance,    cu- 
cumber, 257 

parthenogenesis  in  tobacco,  160 
raspberry  inheritance,  272 
Westgate  and   others,   clover,   seed 
setting,  214 


328 


INDEX 


Wheat,  artificial  crossing  of,  69,  71 

awns  in  wheat,  85 

blooming  in,  69 

breeding,  134,  135 

chaff  characters,  84 

classification  of  species,  77 

disease  resistance,  85,  87,  134 

FI  crosses,  vigor,  43-45 

flower  structure,  34,  70 

hardiness  in,  87 

improvement  by  crossing,  134 

Kanred,  76,  128 

linkage  in  crosses,  79,  80,  84 

Marquis,  136 

natural  crosses  in,  34-36 

Polonicum  crosses,  79 

Red  Rock,  128 

seed  characters,  81-84 

size  characters,  88 

species  crosses,  77,  78 

spike  density,  81 

sterility  in  species  crosses,  79 

T.  dicoccoides,  wild,  78 

wheat-rye  crosses,  35,  106 
White,  endosperm  characters,  maize, 
186 

fruit,  origin,  and  antiquity  of, 
261 

inheritance  in  pea,  236,  239,  240 
Wichura,  species  crosses,  willow,  8 


Wight,  potato  species,  219 
plum  species,  262,  265 
Williams,  ear-to-row,  maize,  198 
selection  for  stiff  straw,  wheat, 

129 
Williams  and  Welton,  ear  characters 

and  yield,  maize,  197 
varietal  crosses,  maize,  203 
weight    of    seed    planted    and 

yield,  124 

Wilson,  inheritance,  potatoes,  222 
Wilson  and  Warburton,  cotton  spe- 
cies, 175 

Witte,  timothy  breeding,  212 
Wittmack,  origin  of  potatoes,  220 
Wolf,  seed  size  increase  due  to  cross, 

maize,  201 
Wood     and     Stratton,     checks     in 

correcting  yield,  53 
probable    error    by    pairing 
method,  58 


X 

Xenia  in  barley,  103 
in  maize,  183 
in  rice,  37 
in  rye,  106 
law  in  maize,  184 


DAY    AND    TO    * 
OVERDUE. 


UNIVERS,TY  OF  CAUFORNIA  LIBRARY 


